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Plato's Theaetetus 

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The Library of Liberal Arts 


The Library of Liberal Arts 


Translated, with a running commentary, by 

Francis MacDonald Cornford 

Late Laurence Professor of Ancient Philosophy and Fellow of 
Trinity College in the University of Cambridge 

The Library of Liberal Arts 

published by 

the BOBB8-MERRILL company, inc. 


Publishers • inoianapolis • New vork 


The present edition is an unabridged reprint of Part One of 
Francis M. Cornford's Plato's Theory of Knowledge, which 
contains the translation, together with the author's running 
commentaries, of both the Theaetetus and the Sophist. 
Originally published by Routledge & Kegan Paul, Ltd., 
London, Plato's Theory of Knowledge is also available in 
this series (LLA 100). 

All Rights Reserved 
Printed in the United States of America 


When the Editor, some eleven years ago, invited me to contribute 
to this series, I offered a translation of the Thcaetetus with a running 
commentary. I have since added the Sophist. Meanwhile the book 
has been announced under the title, Plato's Theory of Knowledge, 
which may seem to promise more than I have performed. My 
object was to make accessible to students of philosophy who can- 
not easily read the Greek text, two masterpieces of Plato's 
later period, concerned with questions that still hold a living 
interest. A study of existing translations and editions has 
encouraged also the hope that scholars already familiar with 
the dialogues may find a fresh interpretation not unwelcome. A 
commentary has been added because, in the more difficult places, 
a bare translation is almost certain, if understood at all, to be 

This danger may be illustrated by a quotation from a living 
philosopher of the first rank : 

■ It was Plato in his later mood who put forward the suggestion 
" and I hold that the definition of being is simply power ". This 
suggestion is the charter of the doctrine of Immanent Law.' 1 
Dr. Whitehead is quoting Jowett's translation. If the reader will 
refer to the passage (p. 234 below), he will see that the words are 
rendered : ' I am proposing as a mark to distinguish real things 
that they are nothing but power/ * A mark of real things may not 
be a ' definition of being '. This mark, moreover, is offered by the 
Eleatic Stranger to the materialist as an improvement on his own 
mark of real things, tangibility. The materialist accepts it, ' having 
for the moment no better suggestion of his own to offer \ The 
Stranger adds that Theaetetus and he may perhaps change their 
minds on this matter later on. Plato has certainly not committed 
himself here to a ' definition of being \ So much could be dis- 

1 A. N. Whitehead, Adventures of Ideas (1933), p. 165. I am not suggesting 
that Dr. Whitehead fundamentally misunderstands the master who has 
deeply influenced his own philosophy, but only pointing out how a profound 
thinker may be misled by a translation. 

* This rendering is itself doubtful, the construction of the words, as they 
stand in the MSS, being obscure and difficult. 


covered from an accurate translation ; but the word ' power ' 
still needs to be explained. It has been rendered by ' potency ', 
' force ', ' Moglichkeit ', ' puissance de relation '. Without some 
account of the history of the word dynamis in Plato's time and 
earlier, the student accustomed to the terms of modern philosophy 
may well carry away a false impression. 

To meet difficulties such as this, I have interpolated, after each 
compact section of the text, a commentary which aims at discovering 
what Plato really means and how that part of the argument is 
related to the rest. There are objections to dissecting the living 
body of a Platonic dialogue. No other writer has approached 
Plato's skill in concealing a rigid and intricate structure of reasoning 
beneath the flowing lines of a conversation in which the suggestion 
of each thought as it arises seems to be followed to an unpre- 
meditated conclusion. In these later dialogues, however, the bones 
show more clearly through the skin; and it is likely that Plato 
would rather have us penetrate his meaning than stand back with 
folded hands to admire his art. An interpolated commentary, 
giving the reader the information he needs when and where he 
needs it, may be preferred to the usual plan of stowing away such 
information in an introduction at the beginning and notes at the 
end. It is not clear why we should be forced to read a book in 
three places at once. This book, at any rate, is designed to be 
read straight through. 

The translation follows Burnet's text, except where I have given 
reasons for departing from it or proposed corrections of passages 
that are probably or certainly corrupt. I have tried to follow 
Plato's own practice of keeping to the current language of educated 
conversation and refusing to allow any word to harden into a 
technical term. The commentary attempts only to interpret 
Plato from his own writings and those of his forerunners and 
contemporaries, and accordingly avoids, so far as possible, the 
misleading jargon of modern philosophy. Terms like ' sub- 
jectivism ', ' relativism ', ' sensationalism ', even when denned, 
often mask ambiguities of thought that are lost sight of as this 
token currency passes from hand to hand. 

At the risk of appearing arrogant or ill-informed, I have, for 
the most part, ignored interpretations which I cannot accept. 
Also I have not loaded the notes with acknowledgments of my 
debts to other scholars. Among works which have most helped 
me I would mention Campbell's editions ; Apelt's translations 
(which contain full bibliographies) ; M. Dies' editions in the Col- 
lection des Universitds de France ; E. Stdlzel, Die Behandlung des 
Erkenntnisproblems bei Plaion (Halle, 1908) ; J. Stenzel, Entwicklung 



der platonischen Dialektik (Breslau, 1917) ; C. Ritter, Neue Unter- 
suchungen uber Platon (Miinchen, 1910) ; V. Brochard, Etudes de 
philosophic ancienne (Paris, 1912) ; and the well-known writings 
of John Burnet and Professor A. E. Taylor. 

Cambridge F. M. C. 





Preface ........ v 

Introduction ....... I 





I54»- I 55i>. 


I 63 A- I 64 B. 





The Introductory Dialogue . 

The Main Dialogue .... 

Introductory Conversation .... 

J. The Claim of Perception to be Knowledge 

Theaetetus identifies knowledge with perception 

Dialectical combination of Theaetetus' position with 
Protagoras' doctrine .... 

Dialectical combination with the Heracleitean 
doctrine of Flux ..... 

Preliminary account of the nature of sense-objects 
and percipients ..... 

Some puzzles concerning size and number . 

Theory of the nature of sense-perception 

Theaetetus accepts the theory of perception 

The claim of perception, so defined, to be infallible 

Interlude. Criticism begins 

Some objections against Protagoras 

Objections to a simple identification of perceiving 
and knowing ...... 

Socrates undertakes to defend Protagoras . 

The Defence of Protagoras 

Interlude ....... 

Criticism of Protagoras' doctrine as extended to all 
judgments ...... 

Restatement of the question : Wherein lies the 
superiority of the wise ? . 

Digression. The contrast of Philosophy and Rhetoric 

Refutation of the Defence of Protagoras 













1790-181B. The extreme Heracleitean position, contrasted with 
Parmenides' denial of all motion and change . 

181B-183C. Criticism of extreme Heracleiteanism . 

1830-184B. Interlude. Socrates declines to criticise Parmenides 

184B-186E. ' Perception is Knowledge ' finally disproved 












//. The Claim of True Judgment to be Knowledge . 109 

Theaetetus states the claim of true judgment . 109 

How is false judgment possible ? . . .110 

False judgment as thinking that one thing (known 
or unknown) is another thing (known or unknown) 1 1 1 

False judgment as thinking the thing that is not . 114 

The apparent impossibility of false judgment as 
mistaking one thing for another . . .116 

One class of mistakes can be explained by taking into 
account memory. The Wax Tablet . .120 

False judgment in general cannot, however, be 
defined as the misfitting of perception to thought 127 

Memory compared to an Aviary, to provide for mis- 
taken judgments not involving perception . 130 

Rejection of ' interchange of pieces of knowledge ' 
as an account of false judgment . . .136 

Conclusion : Knowledge cannot be defined as true 
belief ........ 140 






///. The Claim of True Belief accompanied by an 

account or explanation to be Knowledge . 142 

Socrates states this theory as he has heard it . 142 

The theory criticised for making elements un- 
knowable . . . . . . . 146 

Three possible meanings of ' account ' : (1) Expres- 
sion of thought in speech (irrelevant) . .154 

(2) Enumeration of elementary parts. This will 

not convert a true notion into knowledge . 155 

(3) The statement of a distinguishing mark. This 

will not convert a true notion into knowledge . 158 

Epilogue. All these attempts to define knowledge 
have failed ....... 163 




Since the commentary aims at furnishing the reader with informa- 
tion as the need arises, it will be enough, by way of introduction, 
to indicate the place of the Theaetetus and the Sophist in the series 
of Plato's dialogues, and to define briefly the position from which 
the inquiry starts. 

Our two dialogues belong to a group consisting of the Parmenides, 
the Theaetetus, the Sophist, and the Statesman, As M. Di&s has 
observed, 1 Plato leaves no doubt that the dialogues are meant to 
be read in this order. The Parmenides describes a meeting imagined 
as taking place about 450 B.C. between Socrates, who would then 
be about twenty, and the Eleatic philosophers, Parmenides and 
Zeno. To suppose that anything remotely resembling the con- 
versation in this dialogue could have occurred at that date would 
make nonsense of the whole history of philosophy in the fifth and 
fourth centuries ; and I believe, with M. Dies, that the meeting 
itself is a literary fiction, not a fact in the biography of Socrates. 
No ancient historian of philosophy mistook it for the record of an 
actual event, which, had it occurred, would have been a very 
important landmark. The Theaetetus (183E, p. 101) alludes to this 
meeting, and it is once more recalled in the Sophist (217c, p. 166) 
in terms that can only refer to the Parmenides. The Theaetetus, 
again, ends with an appointment which is kept at the beginning 
of the Sophist ; and the Sophist itself is openly referred to in the 

As for the order of composition, no one doubts that the Sophist 
and the Statesman, which contain one continuous conversation, 
are later than the Theaetetus. In the Theaetetus many critics have 
noticed that the style changes towards the end in the direction 
of Plato's later manner. If that is so, stylometric results based 
on the dialogue as a whole will be misleading. The latter part 
of the Theaetetus, as we have it, may have been finished years 
after the beginning, and the Parmenides may have been composed 
in the interval. On the other hand, we need not suppose any very 
long gap between the completion of the Theaetetus and the com- 
position of the Sophist and the Statesman. 

1 Parminide (1923), p. xii. 



It is now agreed that this group as a whole is earlier than the 
Timaeus, the Philebus, and the Laws, and later than the Meno, 
the Phaedo, and the Republic. The Republic is the centre of a 
group of less technical works, intended, not primarily for students 
of philosophy/ but for the educated public, who would certainly 
not read the Parmenides and would find the Theaetetus and the 
Sophist intolerably difficult. These more popular writings would 
serve the double purpose of attracting students to the Academy 
and of making known to the Greek world a doctrine which, in 
common with most scholars, I hold to be characteristically Platonic. 
Its two pillars are the immortality and divinity of the rational 
soul, and the real existence of the objects of its knowledge — a 
world of intelligible ' Forms ' separate from the things our senses 
perceive. 1 Neither doctrine clearly appears in any dialogue that 
can be dated, on grounds of style, as distinctly earlier than the 
Meno. Both are put forward in the Phaedo in a manner suggesting 
that Plato arrived at them simultaneously and thought of them 
as interdependent. 

The Meno had already announced the theory of Anamnesis : 
that knowledge is acquired, not through the senses or as informa- 
tion conveyed from one mind to another by teaching, but by 
recollection in this life of realities and truths seen and known by 
the soul before its incarnation. Socrates bases this doctrine on 
an account which he believes to be true, 2 learnt from men and 
women who are wise in religious matters and from inspired poets. 
The human soul is immortal (divine) and is purified through a 
round of incarnations, from which, when completely purified, it 
may finally escape. ' So the soul is immortal and has been many 
times reborn ; and since it has seen all things, both in this world 
and in the other, there is nothing it has not learnt. No wonder, 
then, that it can recover the memory of what it has formerly known 
concerning virtue or any other matter. All Nature is akin and 
the soul has learnt all things ; so there is nothing to prevent one 
who has recollected — learnt, as we call it — one single thing from 
discovering all the rest for himself, if he is resolute and unwearying 
in the search ; for seeking or learning is nothing but recollec- 
tion \ 

1 I agree with Mr. J. D. Mabbott (' Aristotle and the ^cupta/xo? of Plato ', 
Classical Quarterly, xx (1926), 72) that the ' separate ' existence of the Forms, 
attacked by Aristotle, is not to be explained away. 

2 Meno 8 1 a, \6yos dXrjd^s, not (xvdos, though the form which contains the 
true account may be mythical. So at Gorgias 52 3 a, he calls the myth of the 
judgment of the dead a \6yos dAi^ifc, though Callicles may think it a fivOos. 
I take the Socrates of the Meno and the Phaedo as stating Plato's beliefs, 
not those of the historic Socrates. 


Socrates goes on to prove this doctrine by experiment. By 
questioning a slave who has never been taught geometry, he elicits 
from him, after several wrong attempts, the solution of a not very 
easy problem of construction. He claims that he has not ' taught ' 
the slave the true belief he now has, any more than the false beliefs 
he produced at first. At the outset the slave had not knowledge ; 
but these beliefs were in him, including the true belief which he 
did not know. They have been ' stirred up in him, as it were in 
a dream ', and if he were questioned again and again in various 
ways, he would end by having knowledge in place of true belief — 
knowledge which he would have recovered out of his own soul. 
This knowledge must have been acquired before birth. ' If, then, 
the truth of things is always in our soul, the soul must be immortal ; 
hence you may confidently set about seeking for and recovering 
the memory of what you do not know, that is to say, do not re- 
member.' Socrates adds that, in some respects, he could not 
defend the whole account ; but he is convinced of the practical 
conclusion, that we shall be the better for believing that we can 
discover truth we do not know. Owing to Plato's dramatic method, 
we cannot fix the extent of Socrates' reservation. It might mean 
that the historic Socrates did not hold this theory, or, more probably, 
that the details of reincarnation, purgatory, and so forth, as described 
by Pindar and others, are ' mythical ' : as such Plato always 
represents them elsewhere. But the reservation does not extend 
to the hypothetical conclusion which Socrates and Meno have both 
accepted : // the truth of things is always in the soul, then the 
soul is immortal. 

Some modern critics, wishing perhaps to transform Plato's 
theory into something that we can accept, reduce the doctrine 
of Anamnesis to a form in which it ceases to have any connection 
with the pre-existence of the soul. But Plato unquestionably 
believed in immortality ; and in the Phaedo, where Recollection is 
reaffirmed, it is the one proof of pre-existence which is accepted 
as satisfactory by all parties to the conversation. 

The doctrine of Recollection marks a complete break with current 
beliefs both about the nature of the soul and about the sources 
of knowledge. The soul was popularly regarded as a mere shadow 
or eidolon, an unsubstantial wraith, that might well be dissipated 
when detached from the body. And if common sense could be 
said to have any view of the common characters called Forms 
(eldrj) in the Socratic dialogues, it would be the empiricist view 
that they are present in sensible things, and that our knowledge 
of them is conveyed through the senses, perhaps by images, like 
the Atomists' eidola, thrown off by material bodies. Among the 



philosophic theories which Socrates, in the Phaedo, says he had 
found unsatisfying is the doctrine ' that it is the brain that gives 
us perceptions of hearing, sight, and smell, and out of these arise 
memory and belief, and from these again, when they have settled 
down into quiescence, comes knowledge '. Plato's break with all 
theories deriving knowledge by abstraction from sensible objects 
carried with it an equally firm repudiation of popular notions of 
the soul as either a flimsy double of the body or a resultant, super- 
vening on the mixture of bodily elements. In other words, the 
' separation ' of the Platonic Forms from any dependence on 
material things went with the separation of the soul which knows 
them from any dependence on the physical organism. The Phaedo 
is designed to plead for both conclusions concurrently. It is not 
claimed that either doctrine is proved ; but it is claimed that if 
the Forms exist and can be known, then the soul is immortal. 
Plato himself believed both ; and his Socrates, unlike the Socrates 
of the earlier dialogues, now uses every resource of eloquence to 
convince his hearers of what he believes but does not know. 

In his opening discourse it is assumed from the outset that the 
soul can exist without the body ; for ' to be dead * is defined as 
meaning ' that the body has come to be separate by itself apart 
from (xcoglg) the soul, and the soul separate by itself apart from 
the body \ * So much might be said of the wraith or shadow-soul 
of popular belief ; but the properties which Socrates goes on to 
ascribe to the separable soul are very different. The contrast is 
not between mind and matter, or even between soul and body as 
commonly understood. The psyche here is what was later called 
by Plato and Aristotle the Reason (vovg), or the spirit, in opposition 
to the flesh. 2 To the flesh belong the senses, and the bodily appetites 
and pleasures. The spirit's proper function is thought or reflection, 
which lays hold upon unseen reality and is best carried on when 
the spirit withdraws from the flesh to think by itself, untroubled 
by the senses. The pursuit of wisdom is a ' loosing and separation 
(xcooiofiog) of the soul from the body ' — a rehearsal of that separa- 
tion called death (67D). 

The effect of this introductory discourse is to establish in the 
reader's mind, before the argument begins, the idea of a complete 
detachment of the thinking self from the body and its senses and 
passions. This idea, though unfamiliar, would be easier for Plato's 
public to grasp than that detachment of Forms from sensible things 

1 64c. In the Gorgias myth (524B), death is already described as the 
' severance (StdAvois) of two things — body and soul — from one another '. 
* Cf. F. M. Cornford, ' The Division of the Soul ', Hibbert Journal (Jan. 



which it is his other purpose to announce clearly for the first time. 
If the reader will forget all that he has learnt about the Forms 
from later writings and put himself in the situation of Plato's 
readers who knew only the earlier dialogues, he will find that he 
is being led, step by step, to recognise the separate existence of 
the Forms. 

The Forms are first mentioned as the objects of the soul's reflec- 
tion, when withdrawn from the senses. All that is pointed out 
here (65 d) is that those entities which were the familiar topics of 
Socrates' conversation are perceived by thought, not by the senses. 
When Socrates and his friends considered, What is Justice ?, they 
were trying to define the Just ' by itself ' (avro), and to discover 
' what it is ' (3 eon) or its ' being ' (ova (a). Any reader of the 
earlier dialogues might agree that Justice, not being a thing 
that can be seen or touched, will be known by pure thought 
when the soul is ' set free from eyes and ears and the body as a 
whole \ 

There follows a long and elaborate defence of Anamnesis, ad- 
dressed to the more difficult task of convincing the reader, on the 
one hand, that the soul has pre-existed, and on the other, that 
his own vague notions of how we first become acquainted with a 
thing like ' Justice itself ' are radically wrong. We not only can- 
not perceive it ; we cannot extract it from any sense-impressions. 
This might be argued more easily in the case of the moral Forms, 
which are obviously not sensible ; but Plato is no less concerned 
with the mathematical Forms. He undertakes to prove that we 
cannot derive our knowledge of Equality from the perception of 
equal things. The same two sticks sometimes appear equal to 
one person and unequal to another ; but no one ever thinks that 
' equals ' are unequal or that Equality is Inequality. The sight 
of nearly equal things causes us to think of Equality, and we judge 
that they fall short of that ideal standard. It is argued that we 
must have obtained knowledge of true Equality before we began 
to use our senses, that is to say, before our birth ; and this carries 
with it the pre-existence of the soul. Whether the argument 
seems sound to the modern reader or not, Anamnesis is accepted 
by all parties and later reaffirmed (92A) ; nor is any doubt ever 
cast upon it in Plato's other works. The upshot is that the Forms 
have an existence separate from things as surely as the spirit has 
an existence separate from the body. 

The next argument is to urge that the soul not only has pre- 
existed, but is by nature indestructible. It is not composed or 
put together out of parts into which it might be dissolved. It is 
reasonable, we are told, to identify incomposite things with things 



that never undergo any sort of change. Now the reader who 
has grasped the distinction between ideal Equality and the nearly 
equal things of sense, will agree that Forms must always be what 
they are and can suffer no kind of change. The many things that 
bear the same names as the Forms are perpetually changing in all 
respects ; and these are the things we see and touch, whereas the 
Forms are unseen. It is thus laid down that there are two orders 
of things : the unseen, exempt from ail change, and the seen, 
which change perpetually. Finally it is argued as probable that 
the soul, which is unseen, most resembles the divine, immortal, 
intelligible, simple, and indissoluble ; while the body most re- 
sembles the human, mortal, unintelligible, complex, and dissoluble. 
The separation of the two worlds or orders of being is here very 
sharply marked. No relation between them is described ; no 
transition from sense to thought is suggested. Even the fact 
that sensible experience may be the occasion of Recollection is 
lost sight of. Socrates recurs to the language of his opening dis- 
course. When the soul uses any ot the senses, it is dragged down 
into the world of change and becomes dizzy and confused. Only 
when thinking by itself can it escape into that other region of 
pure, eternal, and unchanging being. 

Thus, by a series of steps, the reader acquainted with the earlier 
dialogues is led to see that the moral terms which Socrates was 
always discussing belong to a distinct order of realities, and that 
knowledge of them cannot be extracted from impressions of sense. 
Throughout, the separation of the Forms is intertwined with and 
illustrated by the separation of the divine spirit from all dependence 
on the mortal body. The conclusion is that the two doctrines 
stand or fall together. 1 

The separate reality of the Forms created a problem which is 
courageously faced, though not solved, in the later group to which 
our dialogues belong. How are those separate Forms related to 
the things we touch and see in this world of becoming ? The 
Phaedo itself (iooc-d) had indicated that to speak of a thing as 
4 partaking of ' a Form is to use a metaphor that leaves it obscure 
how an eternal and unchanging Form or its character can be ' pres- 
ent in ' or ' shared by ' transient individual things in time and 
space. In the Parmenides Socrates is represented as putting for- 
ward the theory of separate Forms to dispose of Zeno's paradoxical 
antinomies, and as confronted with this very difficulty of participa- 
tion by Zeno's master, Parmenides. It is significant that the great 
founder of the Eleatic school should dominate the discussion here, 
and that a Stranger from Elea should take the lead in the Sophist 

1 Phaedo 76DE, 92D. 



and the Statesman. Parmenides had been the first to raise the 
problem which the theory of Forms was intended to solve. This 
problem had two aspects. In Parmenides' poem it is presented 
chiefly as the problem that arises when a world of real being is 
distinguished from a world of ' seeming ' or appearance, which is 
somehow false and unreal, or, as Parmenides himself declared, 
totally false and unreal. This aspect we shall encounter, as the 
problem of eidola, stated, but not solved, in the Sophist. Par- 
menides had also drawn the corresponding distinction between the 
senses, which profess to reveal appearances, and rational thought 
apprehending true reality. The Theaetetus will formulate and 
examine the claim of the senses to yield knowledge. The discussion 
moves in the world of appearance and proves that, if we try to 
leave out of account the world of true being, we cannot extract 
knowledge from sensible experience. 

The theory of Forms, as stated in the Phaedo, was meant to deal 
with both aspects of the problem bequeathed by Parmenides. The 
eternal and intelligible Forms were to provide rational thought with 
objects of knowledge. The transient existence or ' becoming ' of 
sensible things in the world of appearance was to be grounded in 
the world of true being by some kind of participation ; they were 
thus to be endowed with an ambiguous half -reality, not left, as in 
Parmenides' uncompromising system, totally unsupported. But 
our series of dialogues opens with a trenchant criticism of Plato's 
own theory as giving no intelligible account of the derivation of 
appearances from reality. The discussion starts from Zeno's 
counter-attack on the critics of Parmenides. Zeno had put forward 
a series of arguments, reducing (as he thought) to absurdity their 
defence of the common-sense belief in the existence of a plurality 
of real things. His first argument is quoted : ' If there are many 
things, then they must be both like and unlike.' From both horns 
of the dilemma Zeno deduced what he regarded as impossible con- 
sequences. Socrates replies that no impossibilities result, if you 
recognise ' a Form, Likeness, just by itself ', and another contrary 
Form, Unlikeness. That things which are simply ' alike ' and 
nothing else should be ' unlike ' is no doubt impossible ; but there 
is no difficulty in supposing that individual concrete things should 
partake of both Forms at once and so come to be both like and 
unlike. One thing can have many names, partake of many Forms, 
some of which may be contrary to others. The difficulties dis- 
appear ' if you distinguish the Forms apart by themselves ' and 
realise that individual things partake of them. 

Parmenides' criticisms are directed against this ' separation ' 
(XWQiafiog) of the Forms, on which the Phaedo had laid so much 



stress, 1 and the consequent difficulty of conceiving clearly the 
' participation ' which is to bridge the gulf. Socrates is confronted 
with two questions, which he finds it difficult to answer. 

The first is the extent of the world of Forms. Several classes 
of terms are mentioned, and Socrates is asked if he recognises 
separate Forms for each class, (i) First come the terms which 
had figured in ZenoV dilemmas : Likeness, Unlikeness ; Unity, 
Plurality ; Motion, Rest, etc. 2 To these are added (2) the moral 
Forms, ' Just, Beautiful, Good, etc/. About these two classes 
Socrates has no doubts. (3) The next class contains (a) Forms such 
as ' Man ', ' separate from ourselves and all other men ', and (b) 
Fire and Water. (These terms correspond to the products of divine 
workmanship described in the Sophist 266B (p. 32^) : ' ourselves 
and all other living creatures and the elements of natural things 
— fire, water, and their kindred \ Living organisms and the four 
elements of which all bodies are composed are the two classes of 
things in the physical world with the best claim to represent Forms 
— the models after which the divine creator of the Timaeus works.) 
Socrates says he has often felt some uncertainty about these. 
(Probably they were not contemplated in the early stages of the 
theory, which started with mathematical and moral Forms. But 
they are contemplated in the Timaeus. 3 ) . Last come (4) Hair, Clay, 
Dirt, and other undignified things. (Hair, an organic part of a 
living creature, was one of Anaxagoras' homoeomerous substances ; 
and here it may stand for all organic compounds of the elementary 
bodies. ' Clay ', as Socrates remarks at Theaetetus 147c (p. 22), 
is ' earth mixed with moisture \ Clay and Dirt, as casual mixtures 
of the elements, have the least claim to Forms.) Socrates at first 
replies that he thinks there are no Forms for these undignified 
things ; but he has been troubled with doubts ' whether it may 
not be the same with everything '. Then, fearing to fall into an 
abyss of absurdity, he has returned to the study of Forms of the 
first two classes. Parmenides remarks that when he is older he 

1 Partn. 129D (Socrates), idv ris Siaiprjrat ^topis avra Ka9 y avra. ra elht). 130B 
(Parmenides), avros av ovra> Sirjp-qaai ws \ey€ts, x^pls ptv clSrj avra, drra, ^cupty 
hk rd rovTcov av pL€T€\ovra ; /cat rt 001 8ok€i elvai avrrj ohoiottjs ^cuplj ^S rjfAtis 
ofxotorrjTos exofiev. Here ' the likeness we have ' is distinguished from the 
Form, Likeness itself, as in the Phaedo, ' the tallness in us ' is distinguished 
from Tallness itself. The separate Form is conceived as somehow com- 
municating its character [!&4a, ^op<j>rj) to the individual thing. But how ? 

■ Motion and Rest are included at 129E (cf. Phaedrus 26 id). These terms 
(and the moral Forms) will reappear among the ' common terms ' of Theaetetus 
185c flf. (p. 104), where ' unity and number in general ', ' odd ' and ' even ', 
etc., are added. The mathematical Forms belong to this class. 

* Timaeus 51c (on Forms of the elements) practically quotes Parm. 130D. 



will be more philosophical and pay less regard to vulgar esteem. 
Here this question is dropped. No mention has been made of Forms 
for artificial objects or for sensible qualities like Hot and Cold, 
although ' Hot ' and ' Cold ' had figured in the ideal theory of the 
Phaedo, and the Republic had appeared to recognise a divinely 
created Form of Bedstead. 

What is the extent of the world of Forms ? Plato never answers 
this question. 1 The difficulty arises from the double origin of the 
theory. As Aristotle tells us in his account of Platonism, 2 one root 
was the Socratic inquiry after the definition of ' universals '. Soc- 
rates, who was not concerned with any system of Nature, confined 
himself to the attempt to define moral terms, such as ' Just \ 
Plato (who was concerned with ontology), accepting the Hera- 
cleitean Flux as applied to sensible things, saw that the subject of 
a Socratic definition could not be any sensible thing, since such 
things are in perpetual change and cannot be known ; so he said that 
it must be a separate entity, to which he gave the name ' Form ', 
and that the group of sensible things bearing the same name partake 
of that Form. The underlying assumption here is that every 
common name must have a fixed meaning, which we think of when 
we hear the name spoken : speaker and hearer thus have the same 
object before their minds. Only so can they understand one another 
and any discourse be possible. On this showing, however, all 
common names have the same right to have a Form for their 
meaning ; and so we arrive at the statement (Rep. 5 96 a) : ' we are 
accustomed to assume a single form (or character, eldog) for every 
set of things to which we apply the same name/ We can say : 
' This is hot ', ' This is dirty ', ' This is human ', ' This is just ', 
and so on. If all such statements are on the same footing, we ought 
to recognise a common character or Form for every existing common 
name, and moreover for every entity that might be distinguished 
by a separate name. The world of Forms ought to be indefinitely 
more numerous than the vocabulary of any language. 

But how does this theory look if we start from the other root of 
Platonism — the Pythagorean doctrine of Numbers as the real being 
of all things ? According to Aristotle, Plato conceived the relation 
of things to Forms in the same way as the Pythagoreans conceived 
the relation of things to Numbers : when he said that things 
' partake of ' Forms he was only making a verbal change in their 

1 If Epistle VII, 342A ft. be accepted as genuine, Plato recognised, at the 
end of his life, Forms of mathematical objects, moral terms, every natural 
and artificial body, the four elements, every species of living creature, every 
moral quality, all actions and affections {342D). 

3 Metaph. a, 6. 



statement that things ' represent ' (or embody) Numbers. The Form 
now becomes something more than the meaning of a common name 
— an entity whose metaphysical status Socrates, probably, had 
never inquired into. Socrates had ' no system of Nature ' ; but 
Plato endows the Forms with a r separate ' existence in an intelli- * 
gible world of true being, where they replace the Pythagorean 
Numbers as the reality which appearances are somehow to represent. 
There is no trouble about the mathematical Forms, which are cer- 
tainly distinct from visible and tangible bodies and constitute a 
realm of eternal truth. The moral Forms, again, may stand as 
ideals, never perfectly embodied in human action and character. 
Forms of both these classes can be maintained as eternal things 
which the soul can know (as the Phaedo asserts) without any re- 
course to the bodily senses. Further, when we come to physics, 
we can accommodate the fixed types of natural species and of the 
four elements. But what is to be said of the legion of other common 
names — nouns, adjectives, verbs — which also have fixed meanings ? 
' Clay ' is a common name ; but can physics or metaphysics recognise 
an eternal exemplar of clay and of every distinguishable variety 
of clay ? And what of sensible qualities, like hot and cold ? Is 
Heat or Cold or Redness the sort of object that can be known, 
independently of all sense experience, by a disembodied soul ? Is 
Redness or Hotness an eternally real Form accounting for the 
' becoming ' of red or hot things in the physical world ? Do bodies 
' partake ' of Redness when no one is seeing them, or of Hotness 
when no one feels their heat ? Such may have been the questions 
which embarrassed Plato with the uncertainty confessed by Socrates 
in the Parmenides. The most formidable consequence of recognising 
a Form for every common name would be that no limit could then 
be set to the world of Forms. The unlimited cannot be known, 
and if the Forms are unknowable, their raison d'etre is gone. But 
Plato leaves this question without an answer. 

Parmenides then turns to his second line of criticism : How are 
the separate Forms related to the things that ' partake of ' them ? 

(i) If we press one natural meaning of ' partake ' or ' share ', 
are we to suppose that the Form as a whole is in each of the things, 
or that each thing contains a part of it ? Either supposition is 
absurd. This dilemma can, indeed, be taken as merely an objection 
to certain misleading associations of the word ' partake '.* Many 
things can ' share ' in one Form in the sense that they all have the 
same relation to it. But the question, what that relation can be, 
remains unanswered. 

(2) The suggestion that the Form might be only a ' thought ' in 
1 Cf. G. C. Field in Mind, xxxvi, pp. 87 fJ. 



our minds is decisively rejected. The Form is not a mental existent ; 
it must be an object of thought, of which any number of minds may, 
or may not, think. 

(3) Finally it is suggested that, while the Form has its separate 
reality, what is present here is not the Form, but a copy or image 
of it. One original can have many copies. The relation will then 
be ' likeness \ But this will lead to an infinite regress. If the 
original and the copy are alike, they have a common character, but 
then there will be just as much reason to posit another Form for 
original and copy to partake of as there was to posit the original 
Form for all the copies to partake of. The conclusion is that the 
relation ' partaking ' cannot be reduced to ' likeness ', but we must 
look for some other account of it. The point might be argued thus : 
it may be true that the copy is, at least in some degree, like the 
original ; but that cannot be all that is meant. Likeness subsists 
between any two copies, but we do not say that one copy ' partakes 
of ' another. 

The upshot of all this criticism is that no intelligible account has 
yet been given of the relation between Forms and things ; the 
metaphors will not bear serious scrutiny. Parmenides ends with 
a picture of the ideal world as withdrawn beyond the reach of human 
knowledge. A god might know the Forms, but can we know any- 
thing beyond the things in our world ? On the other hand, Par- 
menides himself acknowledges that the Forms are a necessity of 
thought ; without them philosophic discourse, or indeed discourse 
of any kind, is impossible. This conclusion can only mean that 
the difficulties cannot be insuperable. Plato's intention may be 
to show that he is as aware as any of his critics that they exist, 
and to set his pupils to think about them. 

There is one further problem, mooted by Socrates himself in the 
Parmenides, which is dealt with in the Sophist. This concerns 
the relations of Forms, not to things, but to one another. Socrates 
has just made his point that, if separate Forms are recognised, 
a concrete thing can very well partake both of Likeness and of 
Unlikeness. ' But/ he then adds, ' if you do separate the Forms 
apart by themselves — Likeness and Unlikeness, Plurality and Unity, 
Motion and Rest, and all such things — it would be extraordinarily 
interesting to me if anyone could then show that these Forms 
themselves can be combined and separated ... if one could exhibit 
this same problem as everywhere involved in the Forms themselves/ 
as we have seen it to be in visible things. 1 This challenge is not 
taken up in the early part of the Parmenides. The terms * com- 
bined ' and ' separated ' we shall find in the Sophist used for the 

1 Parm. 129E. 


relations reflected in affirmative and negative true statements about 
Forms. This problem is confined to the ideal world ; it would 
remain if there were no sensible things at all. In such statements 
as ' Likeness exists \ ' Likeness is different from Unlikeness \ the 
meaning consists entirely of Forms ; there is no reference to indivi- 
dual things, and the problem of participation does not arise. The 
question is : How can the unity of the Form, which had been so 
much emphasised, be reconciled with its ' blending ' with other 
Forms ? A Form is ' one being \ Does it, like Parmenides' One 
Being, exclude any sort of plurality, or is a Form both one and 
many ? 

This question is bound up with the methods of Collection and 
Division, which will be illustrated in the Sophist and there identified 
with the dialectical study of the Forms. The early part of the 
Parmenides points forward to the analysis of the blending of Forms 
in that context. Meanwhile, some of the arguments in the later 
part have a positive bearing on this question of their unity. Take 
the bare Eleatic dilemmas : Either a thing is or it is not ; Either 
a thing is one (and not many) or it is many (and not one) ; If the 
One is, the many are not ; if the many are, the One is not. Such 
reasoning must leave us either with a One Being, or Existent Unity, 
excluding all plurality (as in Parmenides' own system), or with a 
plurality having no sort of unity. Now, some of the arguments 
developed in the second part of the Parmenides show that on either 
hypothesis no knowledge or discourse is possible. A bare unity or 
a bare plurality cannot exist or be known or even spoken of. These 
results are deduced by reasoning at least as cogent as Zeno's ; and 
in the Sophist Parmenides' One Being will be criticised on similar 
lines. The arguments point to a positive conclusion : the unity 
of the ' beings ' recognised by Platonism — the whole realm of Forms 
as a ' one being ' and each Form as a ' one being ' — must be shown 
to be consistent with their being also complex and so a plurality. 
The study of Forms in the Sophist will clear up the perplexities and 
paradoxes based by the Eleatics and their successors on the too rigid 
Parmenidean conceptions of Unity and Being, Plurality and Not- 

But before passing to the world of Forms, where the true objects 
of knowledge are to be found, Plato fixes attention, in the Theaetetus, 
on the world of transient becoming and ambiguous appearance, 
revealed by the senses. Writing for students acquainted with the 
great systems of the sixth and fifth centuries, he is now prepared 
to set his own doctrine beside the two opposed philosophies of 
Parmenides and Heracleitus, and to define what he will take, and 
what he will not take, from either. He will also meet the challenge 



of the first and greatest of the Sophists. Protagoras, in conscious 
opposition to Parmenides, had flatly denied that ' what seems to 
men ' — what seems real to our senses and true to our judgment — is 
to be condemned as unreal or false because it disagrees with the 
properties ascribed by Eleatic reasoning to a One Being which we 
can never perceive. Man, declares Protagoras, is the measure of 
all things ; what seems real and true to me is real and true to me ; 
what seems so to you, is so to you. Your perceptions and judgments 
may not agree with mine ; but neither of us can have any ground 
for saying that the other is wrong. Such was the fundamental 
position of that Sophistry which Plato intends to analyse in the 
second of our two dialogues.. The Sophist is the denizen of the 
world of appearances ; they are for him the sole reality. Plato 
himself cannot accept Parmenides' condemnation of appearances 
as totally unreal and of the senses as totally misleading. Accord- 
ingly, the Theaetetus examines afresh the claim of this lower world 
to yield knowledge — a claim that common sense would endorse and 
that Protagoras himself had pressed to the point of declaring that 
it yields the only knowledge we can ever have. 



142A-143C. The Introductory Dialogue 

The main dialogue is prefaced by an introductory conversation 
between Eucleides and Terpsion of Megara, friends of Socrates 
who were present at his death. Plato evidently wished to record 
his affection for Theaetetus, a member of the Academy credited 
with important discoveries in mathematics. Eucleides' account of 
how he came to write the main dialogue is obviously fictitious. 
No such conversation could have taken place in Socrates' lifetime. 
The anonymous commentary on the Theaetetus, 1 believed to date 
from the first or second century of our era, records the existence 
of a second ' rather frigid ' introductory dialogue of about the 
same number of lines, beginning, ' Boy, are you bringing the 
dialogue about Theaetetus ? ' It has been argued that this lost intro- 
duction was probably written by Plato — for why should anyone forge 
such a document ? — and that the obvious occasion for substituting 
the existing one would be the death of Theaetetus. The conclu- 
sion would then be that the main dialogue was at least partly 
written before that event. But it is not likely that the long and 
flattering description in the main dialogue of Theaetetus as a youth 
was written in his lifetime ; and if it was not, the lost introduction 
may be assumed to have been merely a rejected draft which hap- 
pened to be preserved. The whole dialogue — introduction and all 
— may, then, be dated after the fighting near Corinth in 369 B.C. 2 
Theaetetus would then be a little under 50, if he was a lad of 15 
or 16 in the year of Socrates' death, the imaginary date of the 
main dialogue. 

Eucleides. Terpsion 

142. Eucleides. Have you only just come to town, Terpsion ? 
Terpsion. No, some time ago. What is more, I was look- 
ing for you in the market-place and surprised that I could 
not find you. 
Eucl. I was not in the city. 

1 Ed. Diels-Schubart, Berl. Klassikertexte, 1905. 

* The case for this date is fully argued by Eva Sachs, De Theaeteto (Berlin, 
1914). PP- 22 ff. 


THEAETETUS 142a-143c 

142. Terps. Where were you, then ? 

Eucl. On my way down to the harbour I met them 
carrying Theaetetus to Athens from the camp at Corinth. 
Terps. Alive or dead ? 

B. Eucl. Only just alive. He is suffering from severe wounds, 
and still more from having caught the sickness that has 
broken out in the army. 

Terps. The dysentery ? 

Eucl. Yes. 

Terps. How sad that such a man should be so near death ! 

Eucl. An admirable man, Terpsion, and a brave one. 

Indeed, only just now I was hearing warm praise of his 

conduct in the battle. 

Terps. There is nothing strange in that ; it would have 

been much more surprising if he had behaved otherwise. 

C. But why did he not stay here at Megara ? 

Eucl. He was eager to get home. I begged him to stay, 
but he would not listen to my advice. I went some way 
with him, and then, as I was coming back, I recalled what 
Socrates had said about him, and was filled with wonder at 
this signal instance of his prophetic insight. Socrates must 
have met him shortly before his own death, when Theaetetus 
was little more than a boy. They had some talk together, 
and Socrates was delighted with the promise he showed. 
When I visited Athens he repeated to me their conversation, 
d. which was well worth the hearing ; and he added that 
Theaetetus could not fail to become a remarkable man if 
he lived. 

Terps. And apparently he was right. But what was this 
conversation ? Could you repeat it ? 
Eucl. Certainly not, just from memory. But I made 

143. some notes at the time, as soon as I got home, and later 
on I wrote out what I could recall at my leisure. Then, 
every time I went to Athens, I questioned Socrates upon 
any point where my memory had failed and made cor- 
rections on my return. In this way I have pretty well the 
whole conversation written down. 

Terps. True ; I have heard you mention it before, and 
indeed I have always meant to ask you to show it to me ; 
only I have let the matter slip till this moment. Why 
should we not go through it now ? In any case I am in 
need of a rest after my walk to town. 
b. Eucl. For that matter, I should be glad of a rest myself ; 
for I went as far as Erineon with Theaetetus. Let us go 



143B. indoors, and, while we axe resting, my servant shall read 
to us. 
Terps. Very well. 

Eucl. This is the book, Terpsion. You see how I wrote 
the conversation — not in narrative form, as I heard it from 
Socrates, but as a dialogue between him and the other 
persons he told me had taken part. These were Theodorus 
the geometer and Theaetetus. I wanted to avoid in the 
c. written account the tiresome effect of bits of narrative 
interrupting the dialogue, such as ' and I said ' or ' and I 
remarked ' wherever Socrates was speaking of himself, and 
' he assented ' or ' he did not agree ', where he reported the 
answer. So I left out everything of that sort, and wrote 
it as a direct conversation between the actual speakers. 1 
Terps. That was quite a good notion, Eucleides. 
Eucl. Well, boy, take the book and read. 

The Main Dialogue 

The main dialogue is an imaginary conversation, supposed to 
have taken place shortly before, the trial and death of Socrates, 
a date at which Theaetetus would be just old enough to take part. 
He is introduced to Socrates by Theodorus of Cyrene, a distin- 
guished mathematician who has been lecturing on geometry at 

143D-151D. Introductory Conversation 

The opening section characterises the speakers and introduces 
the subject of discussion : the definition of knowledge. For the 
rest, it is concerned with method. Socrates, as in several earlier 
dialogues, dwells on the distinction (which must, it seems, have 
been difficult for the ordinary reader to grasp) between giving a 
number of instances of knowledge and defining the meaning of the 
name ' knowledge ' which applies to them all. He ends by de- 
scribing his own technique. Like the midwife who is past child- 
bearing, Socrates' function is not to produce his own ideas and 
impart them to others, but to deliver their minds of thoughts with 
which they are in labour, and then to test whether these thoughts 
are genuine children or mere phantoms. 

1 Since the Parmenides is composed in the narrative form here rejected as 
tiresome and never again used by Plato, it may be inferred that this intro- 
ductory dialogue was written after the Parmenides. 


THEAETETUS 143d-151d 

Socrates. Theodorus. Theaetetus 

143D. Socrates. If I took more interest in the affairs of Cyrene, 
Theodorus, I should ask you for the news from those parts 
and whether any of the young men there are devoting 
themselves to geometry or to any other sort of liberal study. 
But really I care more for our young men here and I am 
anxious rather to know which of them are thought likely 
to distinguish themselves. That is what I am always on 
the look-out for myself, to the best of my powers, and I 
make inquiries of anyone whose society I see the young men 
ready to seek. Now you attract a large following, as you 
E. deserve for your skill in geometry, not to mention your other 
merits. So, if you have met with anyone worthy of men- 
tion, I should be glad to hear of it. 

Theodorus. Yes, Socrates, I have met with a youth of 
this city who certainly deserves mention, and you will find 
it worth while to hear me describe him. If he were hand- 
some, I should be afraid to use strong terms, lest I should 
be suspected of being in love with him. However, he is 
not handsome, but — forgive my saying so — he resembles 
you in being snub-nosed and having prominent eyes, though 

144. these features are less marked in him. So I can speak with- 
out fear. I assure you that, among all the young men I 
have met with — and I have had to do with a good many — 
I have never found such admirable gifts. The combination 
of a rare quickness of intelligence with exceptional gentle- 
ness and of an incomparably virile spirit with both, is a 
thing that I should hardly have believed could exist, and 
I have never seen it before. In general, people who have 
such keen and ready wits and such good memories as he, 
are also quick-tempered and passionate ; they dart about 
B. like ships without ballast, and their temperament is rather 
enthusiastic than strong ; whereas the steadier sort are 
somewhat dull when they come to face study, and they 
forget everything. But his approach to learning and in- 
quiry, with the perfect quietness of its smooth and sure 
progress, is like the noiseless flow of a stream of oil. It is 
wonderful how he achieves all this at his age. 
Socr. That is good news. Who is his father ? 
Theod. I have heard the name, but I do not remember 
it. However, there he is, the middle one of those three 
c. who are coming towards us. He and these friends of his 
have been rubbing themselves with oil in the portico outside,) 



144c. and, now they have finished, they seem to be coming this 
way. See if you recognise him. 

Socr. Yes, I do ; his father was Euphronius of Sunium, 
just such another as his son is by your account. He was 
a man of good standing, and I believe he left a considerable 
fortune. But I don't know the lad's name. 

D. Theod. His name is Theaetetus, Socrates ; but I fancy 
the property has been squandered by trustees. None the 
less, liberality with his money is another of his admirable 

Socr. You give him a noble character. Please ask him 

to come and sit down with us. 

Theod. I will. Theaetetus, come this way and sit by 


Socr. Yes, do, Theaetetus, so that I may study the char- 

E. acter of my own countenance ; for Theodorus tells me it 
is like yours. Now, suppose we each had a lyre, and 
Theodorus said they were both tuned to the same pitch, 
should we take his word at once, or should we try to find 
out whether he was a musician ? 

Theaet. We should try to find that out. 
Socr. And believe him, if we discovered that he was 
musical, but not otherwise ? 
Theaet. True. 

Socr. And now, if this alleged likeness of our faces is a 
matter of any interest to us, we must ask whether it is a 
145. skilled draughtsman who informs us of it. 
Theaet. I agree. 

Socr. Well, is Theodorus a painter ? 
Theaet. Not so far as I know. 
Socr. Nor an expert in geometry either ? 
Theaet. Of course he is, Socrates ; very much so. 
Socr. And also in astronomy and calculation and music 
and in all the liberal arts ? 
Theaet. I am sure he is. 

Socr. Then, if, in the way of compliment or otherwise, 
he tells us of some physical likeness between us, there is no 
special reason why we should attend to him. 
Theaet. Possibly not. 
B. Socr. But suppose he should praise the mind of either of 
us for its virtue and intelligence. Would there not be good 
reason why the one who heard the other praised should be 
eager to examine him, and he should be equally eager to show 
his quality ? 


THEAETETUS 143d-151d 

145B. Theaet. Certainly, Socrates. 

Socr. Now is the time, then, my dear Theaetetus, for you 
to show your qualities and for me to examine them. I can 
assure you that, often as Theodorus has spoken to me in 
praise of citizen or stranger, he has never praised anyone as 
he was praising you just now. 
Theaet. That is good hearing, Socrates. But perhaps he 

c. was not speaking seriously. 

Socr. No, that would not be like Theodorus. Do not 
try to slip out of your bargain on the pretext that he was 
not serious. We don't want him to have to give evidence 
on oath. In any case no one is going to indict him for 
perjury ; so do not be afraid to abide by your agreement. 1 
Theaet. Well, so it shall be, if you wish it. 
Socr. Tell me, then : you are learning some geometry from 
Theodorus ? 
Theaet. Yes. 

D. Socr. And astronomy and harmonics and arithmetic ? 
Theaet. I certainly do my best to learn. 
Socr. So do I, from him and from anyone else who seems to 
understand these things. I do moderately well in general ; 
but all the same I am puzzled about one small matter which 
you and our friends must help me to think out. Tell me : 
is it not true that learning about something means becoming 
wiser in that matter? 
Theaet. Of course. 

Socr. And what makes people wise is wisdom, I suppose. 
Theaet. Yes. 

e. Socr. And is that in any way different from knowledge ? 
Theaet. Is what different ? 

Socr. Wisdom. Are not people wise in the things of which 
they have knowledge ? 
Theaet. Certainly. 

Socr. Then knowledge and wisdom are the same thing ? 
Theaet. Yes. 

Socr. Well, that is precisely what I am puzzled about: 
I cannot make out to my own satisfaction what knowledge is. 
146. Can we answer that question ? What do you all say ? 
Which of us will speak first ? Everyone who misses 
shall ' sit down and be donkey ', as children say when 

l I question Burnet's punctuation here. The last sentence seems to mean : 
1 Even if he were on oath, there is no one to indict him for perjury, but you 
can keep your agreement without fear of getting him into trouble by not 
coming up to his estimate.' 



6. they are playing at ball ; anyone who gets through without 
missing shall be king and have the right to make us answer 
any question he likes. Why are you all silent ? I hope, 
Theodorus, that my passion for argument is not making me 
ill-mannered, in my eagerness to start a conversation and set 
us all at ease with one another like friends ? 

B. Theod. Not at all, Socrates ; there is nothing ill-mannered 
in that. But please ask one of these young people to answer 
your questions ; I am not at home in an abstract discussion 
of this sort, nor likely to become so at my age. But it is 
just the thing for them, and they have a far better prospect of 
improvement ; youth, indeed, is capable of improving at 
anything. So do not let Theaetetus off ; go on putting 
your questions to him. 

Socr. You hear what Theodorus says, Theaetetus. I do 

C. not think you will want to disobey him ; and it would be 
wrong for you not to do what an older and wiser man bids 
you. So tell me, in a generous spirit, what you think 
knowledge is. 

Theaet. Well, Socrates, I cannot refuse, since you and 

Theodorus ask me. Anyhow, if I do make a mistake, you 

will set me right. 

Socr. By all means, if we can. 

Theaet. Then I think the things one can learn from 

Theodorus are knowledge — geometry and all the sciences 

you mentioned just now ; and then there are the crafts of 

D. the cobbler and other workmen. Each and all of these are 
knowledge and nothing else. 

Socr. You are generous indeed, my dear Theaetetus — 
so open-handed that, when you are asked for one simple 
thing, you offer a whole variety. 
Theaet. What do you mean, Socrates ? 
Socr. There may be nothing in it, but I will explain what 
my notion is. When you speak of cobbling, you mean by 
that word precisely a knowledge of shoe-making ? 
Theaet. Precisely. 
e. Socr. And when you speak of carpentry, you mean just a 
knowledge of how to make wooden furniture ? 
Theaet. Yes. 

Socr. In both cases, then, you are defining what the craft 
is a knowledge of ? 
Theaet. Yes. 

Socr. But the question you were asked, Theaetetus, was 
not, what are the objects of knowledge, nor yet how many 


THEAETETUS 143d-151d 

146E. sorts of knowledge there are. We did not want to count 
them, but to find out what the thing itself — knowledge — is. 
Is there nothing in that ? 
Theaet. No, you are quite right. 
147. Socr. Take another example. Suppose we were asked 
about some obvious common thing, for instance, what clay 
is ; it would be absurd to answer : potters' clay, and oven- 
makers' clay, and brick-makers' clay. 
Theaet. No doubt. 

Socr. To begin with, it is absurd to imagine that our 
answer conveys any meaning to the questioner, when we use 
the word ' clay ', no matter whose clay we call it — the doll- 

B. maker's or any other craftsman's. You do not suppose a 
man can understand the name of a thing, when he does not 
know what the thing is ? 
Theaet. Certainly not. 

Socr. Then, if he has no idea of knowledge, ' knowledge 
about shoes ' conveys nothing to him ? 
Theaet. No. 

Socr. ' Cobblery ', in fact, or the name of any other art has 
no meaning for anyone who has no conception of knowledge. 
Theaet. That is so. 

Socr. Then, when we are asked what knowledge is, it is 
absurd to reply by giving the name of some art. The answer 
is : ' knowledge of so-and-so ' ; but that was not what the 

C question called for. 
Theaet. So it seems. 

Socr. And besides, we are going an interminable way 
round, when our answer might be quite short and simple. 
In this question about clay, for instance, the simple and 
ordinary thing to say is that clay is earth mixed with 
moisture, never mind whose clay it may be. 
Theaet. It appears easy now, Socrates, when you put it 
like that. The meaning of your question seems to be the 
same sort of thing as a point that came up when your 

D. namesake, Socrates here, and I were talking not long ago. 1 
Socr. What was that, Theaetetus ? 
Theaet. Theodorus here was proving to us something 
about square roots, namely, that the sides (or roots) of 
squares representing three square feet and five square feet 

1 The following passage is discussed and interpreted by Sir Thomas Heath, 
Greek Mathematics, i, 155, and The Thirteen Books of Euclid's Elements, ii, 288. 
Theaetetus' friend, the young Socrates, takes his place as respondent in the 



7D. are not commensurable in length with the line representing 
one foot ; and he went on in this way, taking all the separate 
cases up to the root of seventeen square feet. There for 
some reason he stopped. The idea occurred to us, seeing 
that these square roots were evidently infinite in number, 
to try to arrive at a single collective term by which we 

E. could designate all these roots. 
Socr. And did you find one ? 

Theaet. I think so ; but I should like your opinion. 
Socr. Go on. 

Theaet. We divided number in general into two classes. 
Any number which is the product of a number multiplied 
by itself we likened to the square figure, and we called such 
a number ' square ' or ' equilateral \ 
Socr. Well done ! 
Theaet. Any intermediate number, such as 3 or 5 or any 

I. number that cannot be obtained by multiplying a number 
by itself, but has one factor either greater or less than the 
other, so that the sides containing the corresponding figure 
are always unequal, we likened to the oblong figure, and we 
called it an oblong number. 
Socr. Excellent ; and what next ? 

Theaet. All the lines which form the four equal sides of 
the plane figure representing the equilateral number we 
defined as length, while those which form the sides of squares 

B. equal in area to the oblongs we called ' roots '(surds), as not 
being commensurable with the others in length, but only in 
the plane areas to which their squares are equal. And there 
is another distinction of the same sort in the case of solids. 
Socr. Nothing could be better, my young friends ; I am 
sure there will be no prosecuting Theodorus for false witness. 
Theaet. But, Socrates, I cannot answer your question 
about knowledge as we answered the question about the 
length and the root. And yet you seem to want some- 
thing of that kind ; so, on the contrary, it does appear 
that Theodorus was not speaking the truth. 

c. Socr. Why, if he had praised your powers of running 
and declared that he had never met with a young man 
who was so good a runner, and then you had been beaten 
in a race by the greatest of runners at the height of his 
powers, do you think that his praise would have been any 
the less truthful ? 
Theaet. No, I don't. 
Socr. Well, as I said just now, do you fancy it is a small 


THEAETETUS 143d-151d 

148c. matter to discover the nature of knowledge ? Is it not 
one of the hardest questions ? 
Theaet. One of the very hardest, I should say. 
Socr. You may be reassured, then, about Theodoras' 

D. account of you, and set your mind on finding a definition 
of knowledge, as of anything else, with all the zeal at your 

Theaet. If it depends on my zeal, Socrates, the truth 
will come to light. 

Socr. Forward, then, on the way you have just shown 
so well. Take as a model your answer about the roots : 
just as you found a single character to embrace all that 
multitude, so now try to find a single formula that applies 
to the many kinds of knowledge. 

e. Theaet. But I assure you, Socrates, I have often set 
myself to study that problem, when I heard reports of the 
questions you ask. But I cannot persuade myself that 
I can give any satisfactory solution or that anyone has 
ever stated in my hearing the sort of answer you require. 
And yet I cannot get the question out of my mind. 
Socr. My dear Theaetetus, that is because your mind is 
not empty or barren. You are suffering the pains of travail. 
Theaet. I don't know about that, Socrates. I am only 
telling you how I feel. 
149. Socr. How absurd of you, never to have heard that I 
am the son of a midwife, a fine buxom woman called 
Phaenarete ! 

Theaet. I have heard that. 

Socr. Have you also been told that I practise the same 

Theaet. No, never. 

Socr. It is true, though ; only don't give away my secret. 
It is not known that I possess this skill ; so the ignorant 
world describes me in other terms as an eccentric person 
who reduces people to hopeless perplexity. Have you been 
told that too ? 

B. Theaet. I have. 

Socr. Shall I tell you the reason ? 
Theaet. Please do. 

Socr. Consider, then, how it is with all midwives ; that 
will help you to understand what I mean. I dare say you 
know that they never attend other women in childbirth 
so long as they themselves can conceive and bear children, 
but only when they are too old for that. 



149B. Theaet. Of course. 

Socr. They say that is because Artemis, the patroness 
of childbirth, is herself childless ; and so, while she did 
not allow barren women to be midwives, because it is 
c. beyond the power of human nature to achieve skill without 
any experience, she assigned the privilege to women who 
were past child-bearing, out of respect to their likeness 
to herself. 

Theaet. That sounds likely. 

Socr. And it is more than likely, is it not, that no one 
can tell so well as a midwife whether women are pregnant 
or not ? 

Theaet. Assuredly. 

Socr. Moreover, with the drugs and incantations they 
D. administer, midwives can either bring on the pains of 
travail or allay them at their will, make a difficult labour 
easy, and at an early stage cause a miscarriage if they so 

Theaet. True. 

Socr. Have you also observed that they are the cleverest 
match-makers, having an unerring skill in selecting a pair 
whose marriage will produce the best children ? 
Theaet. I was not aware of that. 

Socr. Well, you may be sure they pride themselves on 
e. that more than on cutting the umbilical cord. Consider 
the knowledge of the sort of plant or seed that should be 
sown in any given soil ; does not that go together with 
skill in tending and harvesting the fruits of the earth ? 
They are not two different arts ? 
Theaet. No, the same. 

Socr. And so with a woman ; skill in the sowing is not 
to be separated from skill in the harvesting ? 
Theaet. Probably not. 

150. Socr. No ; only, because there is that wrong and ignorant 
way of bringing together man and woman which they 
call pandering, midwives, out of self-respect, are shy even 
of matchmaking, for fear of falling under the accusation 
of pandering. Yet the genuine midwife is the only suc- 
cessful matchmaker. 
Theaet. That is clear. 

Socr. All this, then, lies within the midwife's province ; 

but her performance falls short of mine. It is not the 

way of women sometimes to bring forth real children, 

b. sometimes mere phantoms, such that it is hard to tell the 


THEAETETUS 143d-151d 

150B. one from the other. If it were so, the highest and noblest 
task of the midwife would be to discern the real from the 
unreal, would it not ? 
Theaet. I agree. 

Socr. My art of midwifery is in general like theirs ; the 
only difference is that my patients are men, not women, 
and my concern is not with the body but with the soul 
that is in travail of birth. And the highest point of my 
c. art is the power to prove by every test whether the off- 
spring of a young man's thought is a false phantom or 
instinct with life and truth. I am so far like the midwife, 
that I cannot myself give birth to wisdom ; and the common 
reproach is true, that, though I question others, I can 
myself bring nothing to light because there is no wisdom 
in me. The reason is this : heaven constrains me to serve 
as a midwife, but has debarred me from giving birth. 
D. So of myself I have no sort of wisdom, nor has any dis- 
covery ever been born to me as the child of my soul. Those 
who frequent my company at first appear, some of them, 
quite unintelligent ; but, as we go further with our dis- 
cussions, all who are favoured by heaven make progress 
at a rate that seems surprising to others as well as to them- 
selves, although it is clear that they have never learnt 
anything from me ; the many admirable truths they bring 
to birth have been discovered by themselves from within. 
But the delivery is heaven's work and mine. 
e. The proof of this is that many who have not been conscious 
of my assistance but have made light of me, thinking it 
was all their own doing, have left me sooner than they 
should, whether under others' influence or of their own 
motion, and thenceforward suffered miscarriage of their 
thoughts through falling into bad company ; and they have 
lost the children of whom I had delivered them by bring- 
ing them up badly, caring more for false phantoms than 
for the true ; and so at last their lack of understanding 

151. has become apparent to themselves and to everyone else. 
Such a one was Aristides, son of Lysimachus, and there 
have been many more. When they come back and beg 
for a renewal of our intercourse with extravagant protesta- 
tions, sometimes the divine warning that comes to me 
forbids it ; with others it is permitted, and these begin 
again to make progress. In yet another way, those who 
seek my company have the same experience as a woman 
with child : they suffer the pains of labour and, by night 



151. and day, are full of distress far greater than a woman's ; 

B. and my art has power to bring on these pangs or to allay 
them. So it fares with these ; but there are some, Theae- 
tetus, whose minds, as I judge, have never conceived at 
all. I see that they have no need of me and with all good- 
will I seek a match for them. Without boasting unduly, 
I can guess pretty well whose society will profit them. I 
have arranged many of these matches with Prodicus, and 
with other men of inspired sagacity. 

And now for the upshot of this long discourse of mine. 
I suspect that, as you yourself believe, your mind is in labour 
with some thought it has conceived. Accept, then, the 

C. ministration of a midwife's son who himself practises his 
mother's art, and do the best you can to answer the ques- 
tions I ask. Perhaps when I examine your statements I 
may judge one or another of them to be an unreal phantom. 
If I then take the abortion from you and cast it away, 
do not be savage with me like a woman robbed of her 
first child. People have often felt like that towards me 
and been positively ready to bite me for taking away 
some foolish notion they have conceived. They do not see 
that I am doing them a kindness. They have not learnt 

D. that no divinity is ever ill-disposed towards man, nor is 
such action on my part due to unkindness ; it is only that 
I am not permitted to acquiesce in falsehood and suppress 
the truth. 

So, Theaetetus, start again and try to explain what 
knowledge is. Never say it is beyond your power ; it will 
not be so, if heaven wills and you take courage. 

Midwifery and Anamnesis. — It is significant that this introduc- 
tory conversation runs closely parallel with the first part of an 
earlier dialogue, the Meno. When asked to define Virtue, Meno 
made the same mistake as Theaetetus, offering a list of virtues 
instead of a definition of the ' single form ' common to them all. 
Socrates' illustration of a correct definition (' Figure ' means ' the 
boundary of a solid ') was drawn, as here, from mathematics. 
Meno's complaint that Socrates does nothing but reduce others to 
perplexity is here quoted by Socrates himself. 1 At this point there 
follows in the Theaetetus the description of the art of midwifery, 
in the Meno the theory of Anamnesis — that all learning is the 

1 Meno 79E, tJkovov . . . on av ovhkv aAAo t) avros T€ arroptis Kal tovs aXAovs 
TTOictj avopilv. Theaet. 149A, Acyouai . . . on . . . aroirwraros (anopwraros 
conj. Stallb.) ci/u teal ttoio> tovs dvdpumovs airoptiv. 


THEAETETUS 143d-151d 

recovery of latent knowledge always possessed by the immortal 
soul. 1 One of the few valuable remarks of the Anonymous Com- 
mentator is upon the equivalence of these two conceptions : 
' Socrates calls himself a midwife because his method of teaching 
was of that kind ... for he prepared his pupils themselves to 
make statements about the subject by unfolding their natural 
ideas and articulating them, in accordance with the doctrine that 
what is called learning is really recollection, and that every human 
soul has had a vision of reality, and needs, not to have knowledge 
put into it, but to recollect ' (on 149A). There is some evidence 
that the historic Socrates professed the art of a spiritual midwife 2 ; 
but Anamnesis appears first in the middle group of dialogues and 
provides the link between two Platonic doctrines : the eternal 
nature of the human soul and the ' separate ' existence of Forms, 
the proper objects of knowledge. The probable inference is that 
Anamnesis was a theory which squared the profession and practice 
of Socrates with Plato's discovery of the separately existing Forms 
and his conversion from Socratic agnosticism to a belief in im- 

Now the Theaetetus will later have much to say about memory. 
Why is there no mention of that peculiar impersonal memory of 
knowledge possessed before birth ? There is no ground for sup- 
posing that Plato ever abandoned the theory of Anamnesis. It 
cannot be mentioned in the Theaetetus, because it presupposes 
that we know the answer to the question here to be raised afresh : 
What is the nature of knowledge and of its objects ? For the 
same reason all mention of the Forms is, so far as possible, excluded. 
The dialogue is concerned only with the lower kinds of cognition, 
our awareness of the sense- world and judgments involving the per- 
ception of sensible objects. Common sense might maintain that, 
if this is not all the ' knowledge ' we possess, whatever else can 
be called knowledge is somehow extracted from such experience. 
The purpose of the dialogue is to examine and reject this claim 
of the sense-world to furnish anything that Plato will call ' know- 
ledge \ The Forms are excluded in order that we may see how 
we can get on without them ; and the negative conclusion of the 
whole discussion means that, as Plato had taught ever since the 
discovery of the Forms, without them there is no knowledge at all. 

The Marks of Knowledge. — The Greek word for ' knowledge ', 
like the English, can mean either the faculty of knowing or that 
which is known. The problem here is to define the faculty or 
function of knowing, though it cannot be defined without reference 

1 On Anamnesis, see Introd., p. 2. a Aristophanes, Clouds 137. 



to its objects. If we are to decide whether sensation or perception 
or belief is to be called knowledge or not, we must assume certain 
marks that any candidate for the title must possess. As Plato 
argues elsewhere, 1 it is a question partly of the inherent qualities 
of our state of mind, partly of the nature of the objects, and from 
differences in the state of mind differences in the objects can be 
inferred. In Republic V this is applied to the contrast between 
Knowledge (yvcbocg) and Opinion (do£a), in the wide sense which 
covers all acquaintance with sensible things and judgments about 
them. The states of mind differ in that knowledge is infallible, 
whereas opinion may be true or false. It is inferred that the 
objects of knowledge must be completely real and unchanging, while 
the objects of opinion are not wholly real and are mutable. 

So here, these two marks of knowledge are assumed at the outset. 
Socrates will point out that Theaetetus' identification of perception 
with knowledge means that perception is infallible and has the 
real for its object (152c). Hence what the dialogue proves is that 
neither sense-perception nor judgment (do£a) of the types con- 
sidered possesses both these marks. We shall find that perception, 
although with due qualifications it may be called infallible, has 
not the real for its object. 

The discussion falls into three main parts, in which the claims 
of (I) Perception, (II) True Opinion or Belief, (III) True Belief 
accompanied by an ' account ' or explanation of some kind, are 
examined and rejected. 

I. The Claim of Perception to be Knowledge 

151D-E. Theaetetus identifies knowledge with perception 

Plato naturally starts with the position of common sense, that 
knowledge comes to us from the external world through the senses. 
In his own view this is the lowest type of cognition ; he works 
upwards from beneath towards the world of intelligible objects, 
so as to see whether we can find knowledge at these lower levels 
without having to cross the boundary between the sensible and 
the intelligible. 

15 id. Theaet. Well, Socrates, with such encouragement from 
a person like you, it would be a shame not to do one's 
best to say what one can. It seems to me tha t one who 
E. kno ws something is perceiving the .t h ing he, knows , and^ 
so far ^ I can see at present, knowledge is nothing jr ut 

Socr. Good ; that is the right spirit in which to express 

1 Rep. V, 477 ft. 

THEAETETUS 151e-152c 

151E. one's opinion. But now suppose we examine your offspring 
together, and see whether it is a mere wind-egg or has 
some life in it. Perception, you say, is knowledge ? 
Theaet. Yes. 

The Meaning of ' Perception '. — In ordinary usage aesthesis, 
translated ' perception ', has a wide range of meanings, including 
sensation, our awareness of outer objects or of facts, 1 feelings, 
emotions, etc. At 156B the term is said to cover perceptions 
(sight, hearing, smell), sensations of heat and cold, pleasures and 
pains, and even emotions of desire and fear. All these are seated 
in the sentient part of the soul, inseparably associated with the 
body. 2 Theaetetus' words, ' one who knows something is per- 
ceiving the thing he knows ', su ggest that he i s chiefly thinkin g 
of perception of external objects, and the criticism which follows 
n arrows down the word "to that sense or at least treats "sense- 
perception of external objects as typical of all aesthesis. The only 
case a nalysed is vision. 

151E-152C. Dialectical combination of Theaetetus* position with 
Protagoras' doctrine 

Socrates at once starts upon the dialectical treatment of Theaete- 
tus' suggestion. ' Dialectical ' has some implications which may 
escape the modern reader. He will readily understand that dia- 
lectic means a co-operative inquiry carried on in conversation be- 
tween two or more minds that are equally bent, not on getting 
the better of the argument, but on arriving at the truth. A tenta- 
tive suggestion (' hypothesis ') put forward by one speaker is cor- 
rected and improved until the full meaning is clearly stated. The 
criticism that follows may end in complete rejection or lead on to 
another suggestion which (if the examination has been skilfully 
conducted) ought to approach nearer to the truth. 3 In the present 
instance three successive suggestions will be made, and all will be 

A less familiar feature of dialectic is the treatment of current 

1 Aristotle, Politics 1276A, 29 : Babylon was so huge that when the city 
fell, it was three days before some of the inhabitants became aware of the 
event (alodeodai). At de anima, 427A, 19, Aristotle remarks that thinking 
and the exercise of intelligence are commonly regarded as ' a sort of percep- 
tion ', for in both the soul discerns and becomes acquainted with something 
that exists. 
. 2 Timaeus 42A. 

8 Cf. Theaet. 187B, where Socrates, after Theaetetus' first definition of 
knowledge has been rejected, says : ' Blot out all we have been saying and 
see if you can get a clearer view from the position you have now reached. 
Tell us once more what knowledge is.' 



views, whether popular or philosophic. Aristotle regularly begins 
his treatises with a review of received opinions, proceeding on the 
avowed assumption that any belief accepted by common sense or 
put forward by wise men is likely to contain some measure of truth, 
however faultily expressed. It is the business of dialectic, by sym- 
pathetic comparison and criticism, to elicit these contributions and 
to make the best that can be made of them. It is here that a modern 
reader is likely to be misled. He will expect a philosopher who 
criticises another philosopher to feel himself bound by the historical 
question, what that other philosopher actually meant. But neither 
Plato nor Aristotle is writing the history of philosophy ; rather they 
are philosophising and concerned only to obtain what light they 
can from any quarter. We can never assume, as a matter of course, 
that the construction they put upon the doctrines of other philos- 
ophers is faithful to historic fact. 

Plato's procedure here is a classic example of dialectical method. 
The first object is to bri n g to l ight_t he full meanin g of t he bare 
statement that perception is knowledge. This is accomplished in 
the first section of the argument ending (i6oe) with the remark that 
Theaetetus' child has now been brought to birth. Socrates also 
says that, in the course of elucidation, Theaetetus* identification of 
perception with knowledge ' has turned out to coincide ' with the 
Heracleitean doctrine that all things are in motion and the Pro- 
tagorean dictum that man is the measure of all things. What has 
really happened is that Plato has given an account of the nature of 
perception which involves elements taken from Protagoras and 
Heracleitus — elements that Plato himself accepts as true when they 
are guarded and limited with the necessary qualifications. Pro- 
tagoras and Heracleitus, in fact, are handled as if they were parties 
to the discussion who could be laid under contribution. 1 Having 
adopted these elements of truth, Plato will be free, in the subsequent 
criticism, to point out what he will not accept from Protagoras and 
the extreme Heracleiteans. 

151E. Socr. The account you give of the nature of knowledge 
152. is not, by any means, to be despised. It is the same that 
was given by Protagoras, though he stated it in a somewhat 
different way. He says, you will remember, that ' man is 
the measure of all things — alike of the being of things that 
are and of the not-being of things that are not \ No doubt 
you have read that. 

1 Compare Socrates' proposal to ' follow up ' the meaning of Protagoras' 
saying (i-rraKoXovd-qaco^ev avrat 152B) with Aristotle, Met. 985A, 4 : ' If we 
were to follow out (aKoXovdolrj) Empedocles' view and interpret it according 
to its meaning and not to its lisping expression, we should find . . .' 


THEAETETUS 151e-152c 

152. Theaet. Yes, often. 

Socr. He puts it * in this sort of way, doesn't he ? — that 
any given thing 'is to me such as it appears to me, and is 
to you such as it appears to you,' you and I being men. 
Theaet. Yes, that is how he puts it. 

B. Socr. Well, what a wise man says is not likely to be non- 
sense. So let us follow up his meaning. Sometimes, when 
the same wind is blowing, one of us feels chilly, the other 
does not ; or one may feel slightly chilly, the other quite 

Theaet. Certainly. 

Socr. Well, in that case are we to say that the wind in 
itself is cold or not cold ? Or shall we agree with Protagoras 
that it is cold to the one who feels chilly, and not to the 
other ? 

Theaet. That seems reasonable. 

Socr. And further that it so ' appears ' to each of us ? 
Theaet. Yes. 
Socr. And ' appears ' means that he ' perceives ' it so ? 2 

„ Theaet. True . 

c. Socr. ' Appearing ' , 3 then, is the same thing as ' perceiving ', 
in the case of what is hot or anything of that kind. They 
are to each man such as he perceives them. 

Theaet. So it seems. 

Socr. Perception, then, is always of something that is, 
and, as being knowledge, it is infallible. 
Theaet. That is clear. 

The main "point here is stated in Socrates' last speech. ' g ercep ? 
tion is knowledge ' means that percepti on is an infallible apprehen- 
sion uf whd I /57or is real. These are the two marks of knowledge, 
which any "candidate tothe title must possess. 

Theaetetus' statement, so interpreted, certainly does not exhaust 
the meaning of Protagoras' saying. Protagoras' word ' appears ' 
was not confined to what appears real to me in sense-perception ; 
it included, as we shall see later, what appears true to me, what I 

1 Xey€i can mean ' say ' or ' mean '. Since Crat. 386A repeats the formula 
in almost the same words, it may well be a quotation. 

a Ast's conjecture * aio0averai ' for aloQaveodai is confirmed by the Berlin 
papyrus (Diels, Vors. A ii, 228). Cf. 164B, to 8c ye ' ovx 6p<j-' * ovk eiriararai* 

* <f>avraaia is simply the substantive corresponding to the verb <f>aiv€odai, as 
at Soph. 264A (p. 319). We can substitute Theaetetus' word aloddvcrai for 
Protagoras' word ^cu'vcrcu without change of meaning. 



think or judge to be true. 1 On that point Plato will part company 
with Protagoras ; but here, as the qualification ' in the case of what 
is hot or anything of that kind ' indicates, we are taking only the 
relevant application of the doctrine to the immediate perception of 
sensible qualities. 

So far as the infallibility of such perception is concerned, we shall 
see that Theaetetus, Protagoras, and Plato are in agreement. The 
second claim — that what appears to me in perception ' is ', or 
exists, or is real — is at present ambiguous and obscure. Protagoras 
is represented as asserting that when the wind appears cold to me, 
then it is cold to me, however it may appear and be to you. Neither 
of us has any ground for saying that the other is wrong. Each is 
the sole measure or criterion or judge 2 of the ex istence or reality:- 
f Of hi m otjwna,t-hp perceives"! WhaTremains obscure is the meaning 
oi the addition ' to me' or 'for me\ It is probable that Protagoras 
actually meant something different from the construction put upon 
the phrase by Plato for his own purpose. 

Socrates, i n his illustratio n from the wind, introduce s-a- diptinc 

t ion between what may b(Tcalled the sense-object a nd the physic al 
object. There are two different sense-objects, the coolness that 
appears to me and the warmth that appears to you. There is one 
physical object, ' the same wind ' that is blowing. How are the 
two sense-objects related to the single physical object ? Socrates 
asks whether the wind in itself is cold or not. Did Protagoras 
think that the cold and the warmth were qualities (or perhaps 
rather ' things ') both residing in a neutral or public physical object, 
the wind in itself ? The answer suggested by Socrates as Pro- 
tagorean is that the wind is cold to him who feels chilly, but not to 
the other. This is open to several interpretations. The ambiguity 
may be intentional. It would be entirely in accordance with 
dialectical procedure that Plato should ignore what Protagoras 
actually meant and adopt such a construction of his words as would 
contribute to his own analysis of sense-perception. 3 Two possible 
interpretations are as follows. 

(i) The wind in itself is both warm and cold. ' Warm ' and ' Cold ' 
are two~properties which can co-exisfm the same physical object. 
I perceive the one, you perceive the other. ' The wind is cold 
to me ' means that the cold is the property that appears to me or 

1 Diog. L. ix, 15 : ' Protagoras held that the mind consists solely of the 
senses. ' This is probably a false inference from our passage, to which Diogenes 

2 At 178B Plato uses the word Kpir-qpiov, and at 160c Kplrrjs. 

* So the Anonymous Commentator : ' Plato himself knew that Protagoras' 
opinion about knowledge was not the same as Theaetetus'. Hence the words 
Ktvbvveveis . . . rd avra ravra' (15 IE). 


THEAETETUS 1 5 1 e-1 52c 

affects me, though it is not the property that appears to or affects 
you. To say simply that ' the wind is cold ' would naturally be 
taken to imply that it was not warm. But in fact it is both ; so 
we add ' to me ', meaning that I am aware of that property, though 
you are aware of the other. 

(2) The wind in itself is neither warm nor cold. It has neither of 
the properties we severally perceive and is not itself perceptible ; 
it is something that exists outside us and originates my feeling of 
cold and yours of warmth. Our sense-objects, the warm and_the 
cold ^do not exist independently in the public physica lobiect^jjiiL^ 
o nly come i nto existence when the act of perceiving thprrT^ jfes 
jplace. THe wind is cold to me * means that it is not cold in itself 
apart from me, but only gives me the feeling of cold. This cold 
which ' appears ' to me exists for me as a private object of percep- 
tion of which I alone can be aware. The fact that your private 
object is different does not justify you in discrediting my perception 
as false or denying that its object exists, or is real. 1 

It is probable that Protagoras held the first and simpler of these 
two views 2 — that the wind is both warm and cold. The second 
view is an essential feature in the theory of perception presently to 
be advanced as a ' secret doctrine ' — a phrase which implies that it 
was not to be found in Protagoras' book. The first view has not 
broken with the naive realism of common sense, which does not 
doubt that objects have the qualities we perceive. It agrees with 
the doctrine of Protagoras' contemporary Anaxagoras, who taught 
that opposite qualities (or things) such as ' the hot ' and ' the cold ' 
co-exist inseparably in things outside us, and that perception is by 
contraries. ' What is just as warm or just as cold (as the sentient 
organ) neither warms nor cools on its approach ; we do not become 

1 Professor Taylor (Plato, the Man and his Work, 1926, p. 326) thinks that 
the view Plato ascribes to Protagoras ' denies that there is a common real 
world which can be known by two percipients. Reality itself is individual 
in the sense that I live in a private world known only to me, you in another 
private world known only to you. Thus if I say the wind is unpleasantly 
hot and you that it is disagreeably chilly, we both speak the truth, for each 
of us is speaking of a " real " wind, but of a " real " wind which belongs to 
that private world to which he, and only he, has access. No two of these 
private worlds have a single constituent in common, and that is precisely why 
it can be held that each of us is infallible about his own private world. 
Protagoras . . . denies the reality of the " common environment " pre- 
supposed by " intra-subjective intercourse ".' 

This interpretation seems to me much too advanced for Protagoras' date, 
and it contradicts the language of our passage, for it asserts that there are 
two real winds, both private and totally unconnected, whereas Socrates says 
1 when the same wind is blowing ' and asks if ' the wind in itself ' is cold or not. 

* Cf. Brochard, Etudes de Philosophic (Paris, 1926), Protagoras et Democrite. 



aware of the sweet or the sour by means of those qualities them- 
selves ; rather we become aware of the cold by means of the hot, 
of the sweet by means of the sour, according to the deficiency (in us) 
of any given quality ; for he says they are all present in us.' x 
If Protagoras accounted for the same wind feeling cold to me and 
warm to you by the obvious explanation (suggested below at 
158E ff.) that I am already hot, you are cold, the agreement with 
Anaxagoras is clear. Both, again, are at one with Heracleitus, on 
the point that opposites co-exist inseparably. 2 In the main fifth- 
century controversy, the Eastern or Ionian tradition maintained 
that the senses were to be trusted and that things were mixtures of 
the opposites apprehended by sense. The Western tradition in- 
cluded the Eleatics, who denied the evidence of the senses and the 
reality of the opposites. They influenced the Atomists, who said 
that the sensible opposites were ' conventional ' (subjective), not 
properties of the ' real ' atoms. Protagoras' doctrine must have 
been a reply to the Eleatic denial of appearances. It is probable 
that he would maintain that ' hot ' and ' cold ' could co-exist in the 
same real thing without any contradiction. Finally, this view is 
supported by Sextus 3 : ' Protagoras says that matter contains 
the underlying grounds of all appearances, so that matter con- 
sidered as independent can be all the things that appear to all. 
Men apprehend different things at different times according to 
variations in their conditions. One in a normal state apprehends 
those things in matter which can appear to a normal person ; a 
man in an abnormal state apprehends what can appear to the 
abnormal. The same applies to different times of life, to the states 
of sleeping or waking, and to every sort of condition. So man 
proves, according to him, to be the criterion of what exists : every- 
thing that appears to man also exists ; what appears to no man does 
not exist/ If Protagoras held this view, his doctrine was not 
' subject ivist ', and even the term ' relativism ' is dangerously mis- 
leading. For him both the sense-objects exist independently of any 
percipient. The hot and the cold, together with any other proper- 
ties we can perceive in the wind, would constitute ' the wind in 
itself \ Since at this date such properties were regarded as ' things ', 
not as qualities needing some other ' thing ' to possess and support 
them, Protagoras would deny that the wind was anything more 

1 Theophrastus, de S^nsu 28 (on Anaxagoras). 

* Sextus, Pyrrh. Hyp. ii, 63 : ' Because honey seems bitter to some, sweet 
to others, Democritus said it is neither sweet nor sour, Heracleitus that it was 

* Pyrrh. Hyp. i, 21S. Sextus was no doubt influenced by the Theaetetus, 
but appears to have had independent sources also. 


THEAETETUS 152c-153d 

than the sum of these properties, which alone appear to us. ' What 
appears to no man does not exist.' 

The conclusion is that the second view, presently to be formulated 
— the wind in itself is neither warm nor cold till it meets with a 
percipient — is a construction put by Plato himself on Protagoras' 
ambiguous statement. By a legitimate extension of the historic 
doctrine, Plato adapts it to the theory he intends to attribute to 
the ' more refined ' thinkers. 

152C-153D. Dialectical combination with the Heracleitean doctrine 
of Flux 

Plato next introduces another element required for his theory 
of sense-perception. It is drawn from Heracleitus : ' All things 
are in motion.' The suggestion that Protagoras taught this as a 
' secret doctrine ' to his ' pupils ' would deceive no one. Protagoras 
had rlo school ; anyone could attend his lectures and read his books. 
Plato is hinting that the doctrine of universal flux is really drawn 
from another quarter, and he goes on to attribute it to Homer and 
all philosophers except Parmenides. There is no more ground here 
for inferring that Protagoras was a Heracleitean than for inferring 
that Homer was one. Plato's intention is to accept from Heracleitus 
the doctrine that all sensible objects are perpetually changing — a 
fundamental principle of his own philosophy. But to Plato sensible 
objects are not ' all things '. He will later point out that the un- 
restricted assertion, ' All things are always changing ', makes know- 
ledge impossible. 

152c. Socr. Can it be, then, that Protagoras was a very ingenious 
person who threw out this dark saying for the benefit of the 
common herd like ourselves, and reserved the truth as a secret 
doctrine to be revealed to his disciples ? * 
D. Theaet. What do you mean by that, Socrates ? 

Socr. I will tell you ; and indeed the doctrine is a remark- 
able one. It declares that nothing is one thing just by itself, 
nor can you rightly call it by some definite name, nor even 
say it is of any definite sort. On the contrary, if you call 
it ' large ', it will be found to be also small ; if ' heavy ', to 
be also light ; and so on all through, because nothing is one 
thing or some thing or of any definite sort. All the things 
we are pleased to say ' are ', really are in process of becoming, 
as a result of movement and change and of blending one 

1 Truth was the title of Protagoras' book which opened with the famous 
saying. At 160 a Socrates again suggests, ironically, that this Truth may 
have been speaking in cryptic oracles. 



T52E. with another. 1 ^Ve are wr nn^ to speak nf th^m ^ ._' fcgmff » f 
for none of them ever is ; they are always becoming . In 
this matter let us take it that, with the exception of Par- 
menides, the whole series of philosophers agree — Protagoras, 
Heracleitus, Empedocles — and among the poets the greatest 
masters in both kinds, Epicharmus 2 in comedy, Homer in 
tragedy. When Homer speaks of ' Oceanus, source of the 
gods, and mother Tethys *, 3 he means that all things are 
the offspring of a flowing stream of change. Don't you 
understand him so ? 
Theaet. Certainly. 

153. Socr. Who, then, could challenge so great an array, with 
Homer for its captain, and not make himself a laughing-stock ? 
Theaet. That would be no light undertaking, Socrates. 
Socr. It would not, Theaetetus. Their doctrine that 
' being ' (so-called) and ' becoming ' are produced by motion, 
1 not-being ' and perishing by rest, is well supported by such 
proofs as these 4 : the hot or fire, which generates and 
controls all other things, is itself generated by movement and 
friction — both forms of change. These are ways of pro- 
ducing fire, aren't they ? 
b. Theaet. Yes. 

Socr. And further, all living things are born by the same 
processes ? B 
Theaet. Assuredly. 

Socr. Again, the healthy condition of the body is under- 
mined by inactivity and indolence, and to a great extent 
preserved by exercise and motion, isn't it ? 
jv Theaet. Yes. 

Socr. And so with the condition of the soul. The soul 

acquires knowledge and is kept going and improved by learn- 

V ing and practice, which are of the nature of movements. By 

1 The Ionian doctrine that things are mixtures of opposites, considered 
as things that can be blended in various proportions. This figures in 
Empedocles as the composition of complex substances by the juxtaposition 
of opposed elements. Hence Empedocles is included below, though he did 
not hold the Flux doctrine. 

1 Epicharmus, frag. 2 (Diels), eV (jLeraXXayq. Se iravres ivrl navra rov xP° vov » 

3 Quoted Crat. 402B, with Orphic verses and Hesiod. 

4 The proofs may be borrowed from the later Heracleitean literature, and 
partly, perhaps, from medical writers under Heracleitean influence. Cf. 
[Hippocrates] de victu 1. 

6 Was Plato's source acquainted with the primitive analogy, frequently 
noted by anthropologists, between the sexual act and the use of the fire-drill ? 


THEAETETUS 1 52c-l 53d 

153B. inactivity, dullness, and neglect of exercise, it learns nothing 
c. and forgets what it has learnt. 
Theaet. True. 

Socr. So, of the two, motion is a good thing for both soul 
and body, and immobility is bad. 
Theaet. So it appears. 

Socr. Need I speak further of such things as stagnation in 
air or water, where stillness causes corruption and decay, 
when motion would keep things fresh ; or, to complete the 
argument, press into its service that ' golden rope ' in 
D. Homer, 1 proving that he means by it nothing more nor less 
than the sun, and signifies that so long as the heavens and 
the sun continue to move round, all things in heaven and 
earth are kept going ; whereas if they were bound down and 
brought to a stand, all things would be destroyed and the 
world, as they say, turned upside down ? 
Theaet. I agree with your interpretation, Socrates. 

In this Heracleitean doctrine two propositions may be dis- 

(1) The first is essential to the Heracleitean harmony of opposites : 
No contrary can exis t apart from its o wn contrary. This is the mean- 
ing here given to the statement that ' nothing- is one thing just by 
itself '. You cannot give it the name of any contrary, such as 
' large - or ' heavy ', without also calling it ' small ' or ' light \ 
Plato makes this ' blending of opposites ' characteristic of the par- 
ticular things of sense. Thus at Rep. 479 A ff. against the lover of 
appearances who believes only in the many beautiful things, not in 
Beauty itself, it is urged that there is no one beautiful thing that 
will not also appear ugly, and that large or heavy things have no 
better claim to be so called than to be called small or light. This 
inseparability of opposites was, as we saw, held also by Protagoras, 
if it is true that he regarded the wind in itself as both hot and cold. 
Here is the real point of contact between Protagoras, Heracleitus, 
and Plato. 

(2) The second proposition is : All the thingswe speak &f_as 
having ' b eing_ ^ never really * are , but are aiwavsTriproce ss of 
becoming, as t he result of motion. There is no obvious reason why 
TrotagorassEould hold this, any more than Anaxagoras did. 2 But 

1 Socrates, in the vein of sophistic interpretation of the poets, misuses the 
passage where Zeus challenges the gods to see if they can drag him down by 
a golden rope. If he chose to pull his hardest, he could drag them all up with 
earth and sea as well. Iliad viii, 18 ff. 

1 Sextus indeed (Pyrrh. Hyp. i, 217 = Vors. 74A, 14) says Protagoras held 
that ' matter is in flux ' (t^v vXrjv p€vorr t v etvcu), and as it flows waste is 



as applied to sensible things, Plato accepted the Heracleitean thesis. 1 
The real being of intelligible objects is always the same, never 
admitting any kind of modification ; but the many things perceived 
by sense never remain in the same condition in any respect. 2 This 
principle Plato now builds into his doctrine of sense-perception. 
The effect is to modify Protagoras' statement, ' I am the measure of 
what-^-r-what^^pe^rs^to me x'jrto me \ For this ' i s 'we now 
s ubstitute^becomes '. In the sphere of perception I am the measure 
of what becomes, but never is ; and the Protagorean claim (152c) 
that ' perception is always of what is ' gives place to the Platonic 
doctrine : Perception is^ways_oJ^whatjsJn process of ^becoming. 

153D-154B. Preliminary account of the nature of sense-objects and 

The next step is to give a precise meaning to the words ' for me ' 
or ' to me ' in the Protagorean formula, ' What appears to me is 
for me or tome', and the Platonic formula, ' What I perceive becomes 
for me or to me'. The interpretation now to be given is : The 
quality I perceive (my sense-object) becomes or arises at the moment 
when it is perceived and only for a single percipient ; it has no 
enduring independent existence in the physical object at other 
times. Here again, if we are right, Plato is going beyond Protagoras. 

t-^153D. Socr. Think of it, then, in tins way. First, to take the case 
of the eyes, you must conceive that what you call white colour 
has no being as a distinct thing outside your eyes nor yet 
inside them, nor must you assign it any fixed place. Other- 
E. wise, of course, it would have its being in an assigned place 
and abide there, instead of arising in a process of becoming. 
Theaet. Well, but how am I to think of it ? 
Socr. T^ n<; follow ™it our recent statement and lav.i t 
down that there is no single thing that is in and by itself. 3 

repaired by additions and our sensations are modified according to various 
times of life and bodily conditions. This may mean no more than the constant 
waste in our bodies repaired by nutrition (cf. Symp. 207D), an alternation of 
hunger and repletion which would modify the pleasures of eating. Sextus' 
source is unknown. He may have been misled by Socrates' dialectical 
inclusion of Protagoras among the adherents of the Flux doctrine (152E). 

1 At., Met. A 6,987a, 32 : ' For having in his youth first become familiar 
with Cratylus and with the Heraclitean doctrines (that all sensible things 
are ever in a state of flux and there is no knowledge about them), these views 
he held even in later years ' (Ross trans.). 

1 Phaedo 78D. 

■ This rather bare and obscure statement here receives a new meaning. 
At 152D tv pkv avro KaO* avro ovbev icniv meant that no quality (contrary) 
exists without its contrary. This was compatible with the independent 
existence of qualities. Now nrjhiv avro icad* avro tv ov means (as again at 


THEAETETUS 153d-154b 

153E. On that showing we shall see that black or white or any 
colour you choose is a thing that has arisen out of the meeting 
of our eyes with the appropriate motion. What we say ' is ' 

154. this or that colour will be neither the eye which encounters 
the motion nor the motion which is encountered, but some- 
thing which has arisen between the two and is peculiar to 
each several percipient. Or would you be prepared to main- 
tain that every colour appears to a dog or any other creature 
just such as it appears to you ? 
Theaet. Certainly not. 

Socr. Or to another man ? Does anything you please 
appear to him such as it appears to you ? Are you quite 
sure of that ? Are you not much rather sure that it does 
not even appear the same to yourself, because you never 
remain in the same condition ? 
Theaet. I think that is much nearer the mark. 

This preliminary statement, explaining what is meant by ' becomes 
for me ', will be expanded presently. So far, a number of points 
have been very briefly stated. On the side of the object, white 
colour has no permanent being anywhere ; it arises between the 
sense-organ and the physical object when they encounter. Also, it 
is peculiar to the individual percipient in two ways : my sense-object 
is private to me in that no one else can see just what I see, and 
peculiar in that no two people, looking at the same thing, will see 
precisely similar colours ; nor will even the same person at different 
moments, because the condition of his sense-organ will be always 

The above statements refer mainly to the object of perception. 
Ijjceinainsto be added that the subject (which at this stage is identi- 
necL^jHTEe^en^e-ui^ aii, not the mmo!yirni5T^uj 0y--fia^ 
qu alities. nTFca rfgg^pe rmanent qu^litip^ oLiisIriwTTjj^ 
aHaj^jKplf to each new ob ject ; those inherent qualities would 
orJstruct the required modification of the organ. 

154B. Socr. So then, if the thing that we measure ourselves 
against or the thing we touch really were large or white 
or hot, it would never become different the moment it en- 
countered a different person, supposing it to undergo no 
change in itself. And again, if the thing which measures 
itself against the object or touches it were any one of these 
things (large, white, etc.), then, when a different thing 

156E, 8 and 157A, 8) that no thing just by itself (i.e. apart from a percipient) 
has, existing in it, any single quality that we perceive. All such qualities 
arise between it and the percipient at the moment of perception. 



154B. came into contact with it or were somehow modified, it, 
on its side, if it were not affected in itself, would not become 

The expression ' measure ourselves against ' looks at first sight 
like a reference to Protagoras' use of the word in ' Man is the 
measure of all things \ ' Measure ' suggests a constant standard 
of reference ; a measure which itself perpetually varied would 
be useless. But in the present case the subject is no more constant 
than the object, and the common implication of constancy must 
be ruled out. The sense-organ is undergoing perpetual modifica- 
tion no less than the external object, and its fluidity offers no 
obstruction to any fresh affection from without. It appears, how- 
ever, in the next section that the literal measurement of a large 
thing against a small is intended. 

154B-155D. Some puzzles concerning size and number 

If Socrates now proceeded at once to the fuller statement of 
the theory of sense perception, there would be no difficulty. But 
here Plato interpolates some alleged puzzles about what we call 
1 relations ' of size and number, whose relevance to their context 
is by no means obvious. Nor is it easy for us to understand why 
anyone should be perplexed by them. 

154B. Socr. [continues). For as things are, 1 we are too easily led 
into making statements which Protagoras and anyone who 
maintains the same position would call strange and absurd. 
Theaet. How so ? What statements do you mean ? 

c. Socr. Take a simple example, which will make my mean- 
ing quite clear. When you compare six dice with four, 
we say that the six are more than the four or half as many 
again ; while if you compare them with twelve, the six 
are fewer — only half as many — and one cannot say any- 
thing else. Or do you think one can ? 

Theaet. Certainly not. 

Socr. Well then, suppose Protagoras or somebody else 
asks you : Can anything become larger or more otherwise 
than by being increased ? What will you answer ? 
Theaet. I should answer No, if I were to speak my mind 

d. with reference to this last question ; but having regard to 
your previous one, I might reply Yes, to guard against 
contradicting myself. 

1 ' As things are ' (vvv) apparently means ' on the current assumption, 
which has just been denied, that things have permanent qualities '. 


THEAETETUS 154b-155d 

154D. Socr. An excellent answer ; really, you might be inspired. 
But apparently, if you say Yes, it will be like the situation 
in Euripides : the tongue will be incontrovertible, but not 
the heart. 
Theaet. True. 

Socr. Now, if you and I were like those clever persons 
who have canvassed all the thoughts of the heart, we might 

e. allow ourselves the luxury of trying one another's strength 
in a regular sophistical set-to, with a great clashing of 
arguments. But being only ordinary people, we shall 
prefer first to study the notions we have in our own minds 
and find out what they are and whether, when we compare 
them, they agree or are altogether inconsistent. 
Theaet. I should certainly prefer that. 
Socr. So do I ; and, that being so, suppose we look at 
the question again in a quiet and leisurely spirit, not with 
I 55- any impatience but genuinely examining ourselves to see 
what we can make of these apparitions that present them- 
selves to our minds. Looking at the first of them, I sup- 
pose we shall assert that nothing can become greater or 
less, either in size or in number, so long as it remains equal 
to itself. Is it not so ? 
Theaet. Yes. 

Socr. And secondly, that a thing to which nothing is 
added and from which nothing is taken away is neither 
increased nor diminished, but always remains the same in 
Theaet. Undoubtedly. 

b. Socr. And must we not say, thirdly, that a thing which 
was not at an earlier moment cannot be at a later moment 
without becoming and being in process of becoming ? 
Theaet. It certainly seems so. 

Socr. Now these three admissions, I fancy, fight among 
themselves in our minds when we make those statements 
about the dice ; or when we say that I, being of the height 
you see, without gaining or losing in size, may within a 
year be taller (as I am now) than a youth like you, and 

c later on be shorter, not because I have lost anything in 
bulk, but because you have grown. For apparently I am 
later what I was not before, and yet have not become so ; 
for without the process of becoming the result is impossible, 
and I could not be in process of becoming shorter without 
losing some of my bulk. I could give you countless other 
examples, if we are to accept these. For I think you 



155c. follow me, Theaetetus ; I fancy, at any rate, such puzzles 
are not altogether strange to you. 

Theaet. No ; indeed it is extraordinary how they set 
me wondering whatever they can mean. Sometimes I get 
quite dizzy with thinking of them. 
D. Socr. That shows that Theodorus was not wrong in his 
estimate of your nature. This sense of wonder is the mark 
of the philosopher. Philosophy indeed has no other origin, 
and he was a good genealogist who made Iris the daughter 
of Thaumas. 1 ^ 

What is the point of these alleged puzzles ? Though Socrates 
continues : ' Do you begin to understand why these things are 
so, according to the doctrine we are attributing to Protagoras ? ' 
nothing more is said about them in the following context, which 
analyses the process of sense-perception. Socrates leaves Theaetetus 
— and us — to think out these puzzles for ourselves. 

We have just been told that sensible qualities like ' white ' and 
' hot ' have no independent and permanent existence either in 
objects outside us or in our sense-organs. They arise or ' become ' 
between object and organ when the two encounter one another. 
If either object or organ carried about with it permanent qualities, 
this becoming could not occur. And at 154B ' large ' was grouped 
with ' white ' and ' hot ', as if it were a quality on the same footing 
with them ; just as earlier (152 d) ' large ' and ' small ', ' heavy 
and light ', were taken as typical of all contraries. 

The puzzle about the dice is this : When we compare six dice 
with four, we say that the six are more. At another moment, 
when we compare them with twelve, we say they are less. Yet 
the six dice have not increased or diminished in number. Common 
sense, we are told, holds that nothing can be at one moment what 
it was not at another, without becoming ; that a thing cannot 
become greater or less so long as it remains the same in amount ; 
and that it does remain the same in amount, so long as nothing 
is added or subtracted. How, then, can the dice, which have 
remained the same in amount, have become less ? 

It is clear that the difficulty here exists only for one who thinks 
of ' large ' as a quality residing in the thing which is larger than 
something else, with ' small ' as the answering quality residing 
in the smaller thing. If that is so, then, when the large thing is 

1 The Cratylus connects Iris with ctpeiv (40813), and cipeiv (Acyciv) with 
dialectic (398D). So Iris (philosophy) is daughter of Thaumas (wonder). 
Since our passage is unintelligible without the Cratylus, the Theaetetus must 
be the later of the two. 


THEAETETUS 154b-155d 

compared with something larger instead of something smaller, he 
will suppose that it has lost its quality ' large ' and gained instead 
the quality ' small \ By suffering this internal change it will 
have ' become small \ He will then be puzzled when we point 
out that the thing has not altered in size. 

Now when Plato wrote the Phaedo, he certainly regarded ' tall- 
ness ' as an inherent property of the tall person. ' Phaedo is 
taller than Socrates ' was analysed as implying (i) that there are 
two Forms, Tall and Short, of which Phaedo and Socrates severally 
partake ; (2) that Phaedo contains an instance of Tallness (called 
' the tallness in us '), and Socrates an instance of Shortness ; (3) that 
neither the Forms, Tall and Short, nor their instances in us can 
change into their opposites ; and consequently (4) that, if Socrates 
should grow and become taller than Phaedo, the instance of short- 
ness in Socrates must either ' perish ' or ' withdraw ' to give place 
to an instance of tallness. This analysis unquestionably means 
that the person who becomes taller or shorter than another suffers 
an internal change. The example chosen lends itself to this view 
because ' tallness ' was commonly ranked as a physical excellence, 
with beauty, health and strength, and as such it is mentioned 
earlier in the Phaedo. 1 Plato himself shares the ordinary view 
and thinks of tallness as an internal property on the same footing 
as ' hot ' or ' white ', not as standing for a relation between the 
taller person and the shorter. 

Now in our passage, though he repeats his example of Socrates, 
who is now taller than Theaetetus, becoming shorter when Theae- 
tetus outgrows him, he remarks that Socrates will not have changed 
in size. And in the case of the dice it is equally obvious that 
the six dice do not become more or fewer in the sense of increasing 
or diminishing in number. Further, he hints that light on the 
puzzles here is to be drawn from the theory of sense-perception, 
which tells us that an object can ' become white ' for a percipient 
without undergoing any internal change of quality irrespective of 
a percipient. When we say it ' becomes white for me ' we do 
not mean that it has lost some other colour and gained whiteness 

1 At Phaedo 65D, Tallness (peyeOos), Health, Strength, are instanced as 
Forms, together with Just, Beautiful, Good. That fieyedos means ' tallness ' 
(not ' absolute magnitude ' or ' mathematical magnitude ') is evident from 
Meno 72D. Meno has said that excellence (dpeTrj) in a man is one thing, in 
a woman another. Socrates asks whether this applies to physical excellences : 
are health, tallness (fieyedos), or strength different things in men and in 
women ? Tallness and beauty are coupled at 72B, as in Homer's phrase 
kclAos tc neyas t€. There is no question of the absolute or mathematical 
magnitude of men and women. At Phaedo 65D tallness appears without 
beauty because koX6v has just before been used in its moral sense. 



instead. In itself, apart from a percipient, it is neither white nor 
of any other colour. The change meant by ' becoming white ' 
(for me) is not an internal exchange of qualities, but a change 
that occurs ' between ' the object and the sense-organ. Neither 
of the two carries about with it a permanent property, independent 
of their meeting. 

The inference seems to be that Plato, since writing the Phaedo, 
has given up the view that any of these qualities — hot, white, large 
— is an instance of a Form residing in an individual thing and perish- 
ing or withdrawing out of it when the thing changes. We are now 
to think of the change as falling ' between ' the thing and the 
percipient, not inside the thing. The case of more or less in number 
or size may be introduced partly because it is easier to see in that 
case how a change can occur ■ between ' a thing and a percipient. 1 
The six dice will appear more to me when I compare them with 
four, less when I compare them with twelve, but they have not 
become more or fewer in themselves. This will help us to under- 
stand how a thing can appear or become white for me, without that 
implying that whiteness in it has replaced some other colour. 

It is not safe, however, to infer that Plato has ' abandoned Ideas 
(Forms) of relations ', if that implies that he had drawn any clear 
distinction between relations and qualities. It is rather probable 
that he still sees no important distinction between ' large ' and ' hot ' 
or ' white '. And he nowhere explicitly states that he has aban- 
doned Forms of both relative terms and sensible qualities. 2 

155D-157C. Theory of the nature of Sense-perception 

Socrates now expands the analysis of the process of sense-percep- 
tion, which was briefly announced before the passage on size and 

155D. Socr. (continues). Do you now begin to see the explanation 
of all this which follows from the theory we are attributing 
to Protagoras ? Or is it not yet clear ? 
Theaet. I can't say it is yet. 
Socr. Then perhaps you will be grateful if I help you to 

1 Note that Plato's illustrations are perceptible things — dice, not abstract 
numbers. He is not talking about mathematical ' relations ' between the 
numbers 4, 6, 12. 

2 The treatment by Plato and Aristotle of ' relative terms ' will be further 
discussed below, p. 282. It is one thing to say (with Plato) that ' larger ' and 
4 more ' are relative terms because what is larger or more is always larger 
than something or more than something or 4 in comparison with something ■ 
{npos ti), and another to say (with Campbell) that * size and number are 
wholly relative '. What is number, or any number (say 7), wholly relative to ? 


THEAETETUS 155d-157c 

155D. penetrate to the truth concealed in the thoughts of a man 
e. — or, I should say, of men — of such distinction. 1 
Theaet. Of course I shall be very grateful. 
Socr. Then just take a look round and make sure that none 
of the uninitiate overhears us. I mean by the uninitiate 
the people who believe that nothing is real save what they 
can grasp with their hands and do not admit that actions 
or processes or anything invisible can count as real. 
Theaet. They sound like a very hard and repellent sort 

156. of people. 2 

Socr. It is true, they are remarkably crude. The 
others, into whose secrets I am going to initiate you, are 
much more refined and subtle. Their first principle, on 
which all that we said just now depends, is that the universe 
really is motion and nothing else. And there are two kinds 
of motion. Of each kind there are any number of instances, 
but they differ in that the one kind has the power of acting, 
the other of being acted upon. 3 From the intercourse and 
friction of these with one another arise offspring, endless in 
B. number, but in pairs of twins. One of each pair is some- 
thing perceived, the other a perception, whose birth always 
coincides with that of the thing perceived. Now, for the 
perceptions we have names like ' seeing ', ' hearing ', ' smell- 
ing ', ' feeling cold ', ' feeling hot \ and again pleasures and 
pains and desires and fears, as they are called, and so on. 
There are any number that are nameless, though names have 
been found for a whole multitude. On the other side, the 
brood of things perceived always comes to birth at the 
same moment with one or another of these — with instances 
c. of seeing, colours of corresponding variety ; with instances 
of hearing, sounds in the same way ; and with all the other 
perceptions, the other things perceived that are akin to them. 
Now, what light does this story throw on what has gone 
before, Theaetetus ? Do you see ? 

1 Observe the hints that the coming theory is one that ' we are attributing ' 
to Protagoras, and not to him alone. 

* Like the physical bodies in whose reality they believe, with their essential 
property of hardness and resistance to touch. 

8 The two kinds of motion here meant are : (1) physical objects considered 
as agents with the power of acting upon or affecting our senses ; (2) sense- 
organs, as patients with the capacity of being affected in the way peculiar 
to sensation or perception. Later (156c) both kinds are distinguished, as 
' slow motions (qualitative changes) occurring in the same place ', from the 
rapid movements which pass between them — the offspring mentioned in the 
next sentences. 



156c. Theaet. Not very clearly, Socrates. 

Socr. Well, consider whether we can round it off. The 
point is that all these things are, as we were saying, in 
motion ; but there is a quickness or slowness in their motion. 
The slow sort has its motion without change of place and 
with respect to what comes within range of it, and that is 

d. how it generates offspring ; but the offspring generated are 
quicker, inasmuch as l they move from place to place 
and their motion consists in change of place. As soon, then, 
as an eye and something else whose structure is adjusted 
to the eye come within range and give birth to the white- 
ness together with its cognate perception — things that 
would never have come into existence if either of the two 
had approached anything else — then it is that, as the 

E. vision from the eyes and the whiteness from the thing that 
joins in giving birth to the colour pass in the space between, 
the eye becomes rilled with vision and now sees, and becomes, 
not vision, but a seeing eye ; while the other parent of the 
colour is saturated with whiteness and becomes, on its side, 
not whiteness, but a white thing, be it stock or stone or 
whatever else may chance to be so coloured. 

And so, too, we must think in the same way of the rest — 
' hard ', ' hot ' and all of them — that no one of them has 
157. any being just by itself (as indeed we said before), but that 
it is in their intercourse with one another that all arise in 
all their variety as a result of their motion ; since it is 
impossible to have any ' firm notion ' (as they say) of either 
what is active or what is passive in them, in any single 
case, as having any being. 2 For there is no such thing as 
an agent until it meets with a patient, nor any patient until 
it meets with its agent. 3 Also what meets with something 
and behaves as agent, if it encounters something different 
at another time, shows itself as patient. 4 

The conclusion from all this is, as we said at the outset, 
that nothing is one thing just by itself, but is always in 

b. process of becoming for someone, and being is to be ruled 

1 Taking ovrto 817 (yewwfxeva) as referring forward and explained by the 
following clause with yap. There should be a colon after iorlv (so Dies). 
But perhaps this ovtw 877 should be omitted, with Peipers. 

2 The ambiguity of tfvai ti is discussed below, p. 50. For to uotovv . . . 
avrtLfV, cf. avro rovro avrcov, 163B, 8. 

3 Strictly the present participles mean a thing which is acting, is being 
acted on. It is not denied that there exists beforehand something with the 
power to act or be acted on. 

4 The eyeball can be seen by another eye, the flesh touched, etc. 


THEAETETUS 155d-157c 

157B. out altogether, though, needless to say, we have been 
betrayed by habit and inobservance into using the word 
more than once only just now. But that was wrong, these 
wise men tell us ; and we must not admit the expressions 
* something ' or ' somebody's ' or ' mine ' or ' this ' or ' that ' 
or any other word that brings things to a standstill, but 
rather speak, in accordance with nature, of what is ' be- 
coming \ ' being produced ', * perishing ', ' changing '. For 
anyone who talks so as to bring things to a standstill is 
easily refuted. So we must express ourselves in each 
individual case and in speaking of an assemblage of many — 
c. to which assemblage people give the name of ' man ' or 
' stone ' or of any living creature or kind. 1 

Whose is this theory ? Modern critics usually say that Socrates 
attributes it to ' certain unnamed thinkers ', and many have 
proceeded to identify these with the Cyrenaics. For this there is 
no warrant in the text. The theory is first introduced (152c) as a 
secret doctrine revealed by Protagoras to his disciples. Its funda- 
mental thesis — the flux doctrine — is then ascribed to the whole 
series of philosophers, with the exception of Parmenides, and to 
Homer and Epicharmus. At 155D it is called ' the theory we are 
attributing to Protagoras ', and once more described as a secret 
1 concealed in the thoughts of a man — or rather men — of distinc- 
tion \ Materialists, who identify the real with the tangible and do 
not reckon actions and processes as real at all, are excluded from the 
mystery, which reduces the tangible bodies they believe in precisely 
to actions and processes. 2 ' The others ' 3 are more refined, and 
now their secret doctrine is fully revealed. ' The others ' means 
simply the distinguished men just mentioned, Protagoras himself 
and all the philosophers (except Parmenides, who denied the exist- 
ence of motion) and poets who recognised the flux of all things — 

1 The text is doubtful : ko.1 Zkclotov £u>6v tc koI etSos is hard to construe. 
Does €Kaorov £coov mean ' an individual animal ', cTSo? a ' kind ' of animal ? 
What sort of ' assemblage ' is meant ? Perhaps a physical object considered 
merely as an aggregate of what are commonly regarded as its sensible qualities 
— all the qualities (white, hard, etc.) we should name in describing a stone 
that we saw. The whole theory is confined to the discussion of sensible 
qualities. Cf. Burnet, G.P. i, 241. 

a We shall meet with the materialists again in the Sophist (p. 231, infra). 
Probably no particular school is directly aimed at, though the Atomists who 
identified the real with (essentially tangible) body would come within the 

* Reading iXXot, 8^ with Burnet at 156A, 2. But the reading does not 
affect my argument. 



all who have been wise enough to acknowledge the reality of actions 
and processes. There are no ' unnamed thinkers ' to be identified ; 
nor is there any evidence that any Cyrenaics or other contem- 
poraries existed who held the doctrine of sense-perception here set 

No one would take seriously the suggestion that this very advanced 
theory of the nature of perception and its objects was really taught 
in secret by any of the distinguished philosophers and poets. 
Socrates is, in fact, himself in the act of constructing it by a dialec- 
tical combination of elements borrowed, with important modifica- 
tions and restrictions, from Protagoras and Heracleitus. Jackson * 
pointed out that the theory is not refuted in the sequel, but on the 
contrary taken as a true account of the matter, and that it is 
repeated elsewhere in Plato's writings. He inferred that it origi- 
nated with Plato himself. There is a conclusive argument (not urged 
by Jackson) in favour of this inference. Plato intends to refute 
the claim of perception (in spite of its infallibility) to be knowledge 
on the ground that its objects have no real being, but are always 
becoming and changing and therefore cannot be known. For that 
purpose he is bound to give us what he believes to be a true account 
of the nature of those objects. It would be futile to prove that what 
some other individual or school, perhaps wrongly, supposed to be 
the nature of perception was inconsistent with its claim to yield 
knowledge. Accordingly he states his own doctrine and takes it 
as established for the purposes of the whole subsequent criticism of 
perception. To preserve the dramatic proprieties of dialogue, he 
uses the transparent device of making Socrates state it as a secret 
doctrine of a whole succession of wise men who notoriously had 
never taught anything of the kind. 

Assured that the theory must be Plato's own, we may now look 
at it more closely. Contemporaries must have found it extremely 
daring. The physical objects which yield our sensations and per- 
ceptions are described as actually being ' slow motions \ No 
permanent quality resides in them. The only other thing we know 
about them is that they have the power (dvvapLig) of acting upon our 
organs and (it may be added) upon one another. What we call a 
hot thing is a change that can make us ' feel hot ' and can make 
another thing we call ' cold ' hotter. This change, as opposed to 
locomotion, is a modification or qualitative change. 2 On the other 
side, the subject of perception is here treated as if it were, not the 

1 Journal of Philology xiii, pp. 250 ff. Burnet (Greek Philosophy i, 242) 
agrees with the attribution to Plato. 

2 This is clear from 18 id and Parm. 138B, where it is said that the two kinds 
of change are locomotion (<f>opd) and qualitative change (dAAouoai?). The 
conception of the Swapis will be further discussed below, pp. 234 ff. 


THEAETETUS 155d-157c 

mind, but the sense-organ * — the eye from which issues the stream 
of visual ' fire ' or light (called ' vision ', dipig) — to encounter the 
rapid motion coming from the object. The eye which sees, or the 
flesh which feels, is itself a physical object which can be seen or 
touched, and therefore itself a qualitative change, a ' slow motion 
in the same place '. Thus, before the act of perception takes place, 
there are, on both sides, changes going on all the time in physical 
objects, unperceived and capable of giving rise to actual perceptions. 
But nothing that can properly be called an agent or patient exists 
until the two come within range of one another. 

When they do come within range, the powers of acting and being 
acted upon come into play. Quick motions pass between organ and 
external object. A stream of visual light flows out from the eye 
to meet a stream of light whose structure corresponds in such a 
way that the two streams can interpenetrate one another and 
coalesce. 2 The marriage of these two motions generates seeing 
and colour. Physically, ' the eye becomes rilled with vision ' — 
a mixture of visual fire and the fiery particles coming from the 
object. The external thing ' becomes white ' ; its surface is ' satu- 
rated with whiteness '. This last statement is more difficult ; the 
object is described as affected by the act of sight and acquiring 
colour. The meaning may be that the * flame ' or light belonging 
to the object cannot until this moment be called ' colour ' or 
' white \ At other times the object ought not to be spoken of as 
if it possessed in itself any quality with a fixed name. 

When perception is not taking place, we are finally told, one 
cannot have any ' firm notion ' of either agent or patient as ' having 
any being ' or ' being any definite thing ' (elvairi). The last words 
are ambiguous. ' Being any definite thing ' means having any 
definite quality, such as white. ' Having any being ' means that 
there is strictly no such thing as an agent or patient as such : there 
is nothing that is acting or being acted upon, but only two things 
or changes with a capacity of acting and being acted upon. This 
capacity must imply that my pen and this paper have some differ- 
ence of property when not perceived, which would explain why, when 
I do see them, the pen looks black, the paper white. Plato's point 

1 Later (184B) it will be pointed out that there is a central mind which 
perceives rather through than with the several sense-organs, but this addition 
does not invalidate the present account of the commerce between organs and 

* The Timaeus explains the process in terms of the theory which there 
assigns particles of regular form to each of the four elements. Colours are 
1 a flame streaming off any and every body, having its particles so adjusted 
(av/i/icrpa) to those of the visual current as to excite sensation ' (67c). Cf. 
avynilrpwv here, 156D. The coalescence is described at Tim. 45B ff. See p. 327. 



is that these properties, whatever they are, are always changing, 
however slightly, and that they are not the qualities I perceive — 
my sense-objects — and so should not be called ' black ' or ' white \ 1 

157C-D. Theaetetus accepts the theory of Perception 

In a short interlude, Theaetetus accepts the theory, while Socrates 
disclaims the authorship. 

157c. Socr. {continues). Does all this please you, Theaetetus ? 
Will you accept it as palatable to your taste ? 
Theaet. Really, I am not sure, Socrates. I cannot even 
make out about you, whether you are stating this as some- 
thing you believe or merely putting me to the test. 
Socr. You forget, my friend, that I know nothing of such 
matters and cannot claim to be producing any offspring of 
my own. I am only trying to deliver yours, and to that 
end uttering charms over you and tempting your appetite 
D. with a variety of delicacies from the table of wisdom, 2 
until by my aid your own belief shall be brought to light. 
Once that is done, I shall see whether it proves to have some 
life in it or not. Meanwhile, have courage and patience, 
and answer my questions bravely in accordance with your 

Theaet. Go on with your questioning. 
Socr. Once more, then, tell me whether you like this 
notion that nothing is, but is always becoming, good or 
beautiful or any of the other things we mentioned ? 
Theaet. Well, when I hear you explaining it as you have, 
it strikes me as extraordinarily reasonable, and to be 
accepted as you have stated it. 

The theory so accepted stands henceforth as a satisfactory account 
of that perception which Theaetetus has identified with knowledge. 
The word has now received a clearer meaning, more restricted than 
Theaetetus, perhaps, at first intended. He apparently feels no 
qualm when Socrates slips in the words ' good ' and ' beautiful ', 
as if these qualities were on the same footing with ' hot ' or ' white ' 
or ' large ', and since his identification of knowledge with perception 
implies that there is no knowledge other than perception, he would 
have no right to object. 

1 There is no question here of a ' solipsist epistemology ' or of a relativism 
asserting that, if every sentient creature were annihilated, nothing would exist 

2 The allusion seems to be rather to the fastidious appetite of pregnant 
women than to drugs, which are not ' set before ' the patient to be ' tasted of '. 


THEAETETUS 157e-160e 

157E-160E. The claim of Perception, so defined, to be infallible 

The next section completes the case on behalf of Theaetetus' 
identification of knowledge with perception. At the outset Protag- 
oras' assertion that ' what appears to each man is to him ' was 
construed as meaning that what he perceives has being (at any 
rate ' for him ') and that his perception is infallible. Plato's theory 
of perception has now denied that the object has ' being* apart 
from the percipient, and has interpreted ' is for him " as meaning 
' becomes for him \ This interpretation, though it will finally prove 
fatal to the claim of perception to be knowledge of true reality, 
leaves untouched the claim to infallibility. Socrates, whose present 
business is to make the best of Theaetetus' hypothesis that percep- 
tion is knowledge, now brings forward this latter claim and upholds 
it against the objections commonly based on so-called delusions 
of sense, the unreality of dream images, the vitiated sensations of 
the diseased, and the hallucinations of insanity. 

157E. Socr. Then let us not leave it incomplete. There remains 
the question of dreams and disorders, especially madness and 
all the mistakes madness is said to make in seeing or hearing 
or otherwise misperceiving. You know, of course, that in 
all these cases the theory we have just stated is supposed 
to be admittedly disproved, on the ground that in these 

158. conditions we certainly have false perceptions, and that so 
far from its being true that what appears to any man also 
is, on the contrary none of these appearances is real. 
Theaet. That is quite true, Socrates. 
Socr. What argument, then, is left for one who maintains 
that perception is knowledge, and that what appears to 
each man also ' is ' for him to whom it appears ? 
Theaet. I hesitate to say that I have no reply, Socrates, 
because just now you rebuked me for saying that. Really, 
B. I cannot undertake to deny that madmen and dreamers 
believe what is false, when madmen imagine they are gods 
or dreamers think they have wings and are flying in their 

Socr. Have you not taken note of another doubt that is 
raised in these cases, especially about sleeping and waking ? * 
Theaet. What is that ? 
Socr. The question I imagine you have often heard asked : 

1 The reply our theory will make to dispose of the objection does not begin 
till 158E. Here Socrates makes a sort of preliminary answer : Who is to 
judge between the dreamer's conviction that his experience is real and the 
waking man's, that it is unreal ? 



15 8b. what evidence could be appealed to, supposing we were 
asked at this very moment whether we are asleep or awake 

c. — dreaming all that passes through our minds or talking 
to one another in the waking state. 

Theaet. Indeed, Socrates, I do not see by what evidence 
it is to be proved ; for the two conditions correspond in 
every circumstance like exact counterparts. The conversa- 
tion we have just had might equally well be one that we 
merely think we are carrying on in our sleep ; and when 
it comes to thinking in a dream that we are telling other 
dreams, the two states are extraordinarily alike. 
Socr. You see, then, that there is plenty of room for 

d. doubt, when we even doubt whether we are asleep or awake ; 
and in fact, our time being equally divided between waking 
and sleeping, in each condition our mind strenuously con- 
tends that the convictions of the moment are certainly 
true ; so that for equal times we affirm the reality of the one 
world and of the other, and are just as confident of both. 
Theaet. Certainly. 

Socr. And the same holds true of disorders and madness, 

except that the times are not equal. 

Theaet. That is so. 

Socr. Well, is the truth to be decided by length or shortness 

of time ? 

e. Theaet. No, that would be absurd in many ways. 
Socr. Have you any other certain test to show which of 
these beliefs is true ? 

Theaet. I don't think I have. 

The word aesthesis is here still used in a sense wide enough to 
include awareness of inner sensations and feelings and of dream- 
images. All these are, in Protagoras' phrase, ' things that appear ' 
to me. Since, as Socrates will point out, I cannot be aware and yet 
aware of nothing (160 a), these objects must have some sort of 
existence ; and there is no ground for saying that my direct aware- 
ness of them is ' false \ 

It is true that Theaetetus (158B), instead of keeping to Socrates' 
expressions ' perceptions,' ' what appears ', speaks of the dreamer 
and the madmen as ' thinking ' (do£d£eiv, diavoetoOai) or ' believ- 
ing ' (oieadai) what is false. This is no doubt intentional. It 
stirs in the reader the suggestion that, although there may be no such 
thing as a false awareness of sensation, there is such a thing as 
false belief. But the vital distinction between direct awareness 
and belief is not yet drawn, and Theaetetus, like most people, would 


THEAETETUS 157e-160e 

say indifferently of the dreamer that he ' has the sensation of 
flying ', ' seems to himself to be flying ', and ' imagines or believes 
he is flying \ When the distinction is drawn, the claim of direct 
awareness to be infallible is not shaken. No one can deny that the 
dreamer has just" that experience which he does have. 

After this glimpse of the distinction between sensation or percep- 
tion and belief or judgment, the argument returns to the case of 
' perception ' and is confined to that. Socrates now disposes of the 
popular notion that the healthy or the sane man is the only measure 
of what is or appears — that wine really is in itself sweet because 
it seems sweet to the normal palate, sour only to the unhealthy. 
Since the sense-organ co-operates in producing the sensation, its 
condition at least partly determines the character of the sensation. 
The unhealthy man is not ' misperceiving < a fixed quality inherent 
in the external object, which the normal man perceives as it really 
is. The two percipient organs are different, and these differences 
will necessarily modify the joint product of the marriage of subject 
and object. 

158E. Socr. Then let me tell you what sort of account would be 
given of these cases by those who lay it down that whatever 
at any time seems to anyone is true to him. I imagine they 
would ask this question : ' Tell us, Theaetetus ; when one 
thing is entirely different from another, it cannot be in any 
respect capable of behaving l in the same way as that other, 
can it ? We are not to understand that the thing we speak 
of is in some respects the same though different in others, 
but that it is entirely different/ 

159. Theaet. If so, it can have nothing in common, either in its 
capabilities of behaviour or in any other respect, when it is 
altogether different. 

Socr. Must we not admit, then, that such a thing is unlike 
the other ? 
Theaet. I agree. 

Socr. So if it happens that something comes to be like or 
unlike either itself or something else, we shall say that when 
it is made like it becomes the same, when unlike, different. 
Theaet. Necessarily. 

Socr. And we said earlier that there was no limit to the 
number of things that are active or of things that are acted 
upon by them. 
Theaet. Yes. 

1 By SvvafiLs the capacity of acting or being acted upon, mentioned at 156A, 
is specially meant, though the word has vaguer senses. 



159. Socr. And further, that when one of these is married to a 
succession of different partners, the offspring produced will 
be not the same but different. 

B. Theaet. Certainly. 

Socr. Now let us take you or me or any other instance to 
which the principle applies — Socrates in health and Socrates 
ill : are we to call one of these like the other or unlike ? 
Theaet. You mean : Is the ill Socrates taken as a whole 
like Socrates in health taken as a whole ? 
Socr. You understand me perfectly : that is just what I 

Theaet. Then of course he is unlike. 
Socr. And consequently, inasmuch as he is unlike, a differ- 
ent thing ? 
Theaet. Necessarily. 

c. Socr. And you would say the same of Socrates asleep or 
in any other of the conditions we mentioned ? 
Theaet. Yes. 

Socr. Then any one of the objects whose nature it is to act 
upon something will, according as it finds Socrates well or 
ill, treat me as a different thing ? 
Theaet. Of course it. will. 

Socr. And consequently the pair of us — I who am acted 
upon and the thing that acts on me — will have different 
offspring in the two cases ? 
Theaet. Naturally. 

Socr. Now when I am in health and drink wine, it seems 
pleasant to me and sweet. 
Theaet. Yes. 

Socr. Because, in accordance with the account we accepted 
earlier, agent and patient give birth to sweetness and a 

D. sensation, both movements that pass simultaneously. The 
sensation, on the patient's side, makes the tongue per- 
cipient, while, on the side of the wine, the sweetness, moving 
in the region of the wine, 1 causes it both to be and to appear 
sweet to the healthy tongue. 

Theaet. Certainly that was what we agreed upon. 
Socr. But when it finds me in ill health, to begin with, 
the person it finds is not really the same ; for the one it now 
meets with is unlike the other. 
Theaet. Yes. 

e. Socr. And so this pair — Socrates in this condition and the. 

1 nepl avrov ^epo^ievrj seems to mean, as it were, ' spreading itself over the 
wine ' as whiteness saturated the surface of the thing seen (156E). 


THEAETETUS 157e-160e 

159E. drinking of the wine — produce a different offspring : in the 
region of the tongue a sensation of sourness, and in the region 
of the wine a sourness that arises as a movement there. The 
wine becomes, not sourness, but sour ; while I become, not 
a sensation, but sentient. 
Theaet. Undoubtedly. 

The assertion here that Socrates-ill is a totally different person 
from Socrates-well may seem fallacious. But the whole argument 
is confined within the limits of the earlier account of sense-percep- 
tion. Socrates is for this purpose nothing more than a bundle of 
sense-organs. If these sense-organs are perpetually changing (as 
the theory maintains), then the whole of Socrates is different at any 
two moments. So at i66b Protagoras is made to say that we have 
no right to speak of a single person continuously existing, but only 
of an infinite number, if change of quality is always taking place, 
as it is on our Heracleitean premiss. Socrates is, accordingly, 
justified in drawing the three conclusions that follow : (1) No 
percipient can have the same sensation or perception twice, since 
both subject (organ) and object will be different ; (2) No two 
percipients can have precisely similar sensations or perceptions from 
the same object ; (3) Neither percipient nor sense-object can exist 
independently of the other. These conclusions will yield the final 
result, that no one can challenge the truth of my perception on the 
grounds that he perceives an object different from mine, and that 
that object is a quality which resides in the thing independently of 
either percipient, so that one of us must be ' misperceiving ' it. 

159E. Socr. It follows, then, (1) that, on my side, I shall never 
become percipient in just this way of any other thing ; for to 
a different object belongs a different perception, and in acting 
on its percipient it is acting on * a person who is in a different 
condition and so a different person. Also (2) on its side, 

160. the thing which acts on me can never meet with someone else 
and generate the same offspring and come to be of just this 
quality ; for when it brings to birth another thing from 
another person, it will itself come to be of another quality. 

1 7roici ' is acting on ' (cf. to ttoiovv ifxem the next clause and 160c, 4), not 
' makes him a different person ' ; it finds a different person, since the sense- 
organ is, on our Heracleitean principle, perpetually changing. The agent 
itself is different ; so the combination of a different object and a different 
subject must produce a different sensation. The expression noulv nva for 
1 doing something to a person ' is a slight extension of the common usages, 
€v TTotetv riva, ovk of8' otl XPV^ ^ novels (Ar., Wasps 697), ravra rovrov 
CTToirjaa (Hdt.). 



160. Theaet. That is so. 

Socr. Further, (3) I shall not come to have this sensation 

for myself} nor will the object come to be of such a quality 

for itself. 

Theaet. No. 

Socr. Rather, when I become percipient, I must become 

percipient of something ; for I cannot have a perception 

and have it of nothing ; and equally the object, when it 

b. becomes sweet or sour and so on, must become so to someone : 
it cannot become sweet and yet sweet to nobody. 
Theaet. Quite so. 

Socr. Nothing remains, then, I suppose, but that it and 
I should be or become — whichever expression we are to 
use — for each other ; necessity binds together our existence, 
but binds neither of us to anything else, nor each of us 
to himself ? ; so we can only be bound to one another. 
Accordingly, whether we speak of something ' being ' or 
of its ' becoming ', we must speak of it as being or becoming 
for someone, or of something or towards something ; but 

c. we must not speak, or allow others to speak, of a thing 
as either being or becoming anything just in and by itself. 
That is the conclusion to which our argument points. 
Theaet. Certainly, Socrates. 

Socr. And so, since what acts upon me is for me and 
for no one else, I, and no one else, am actually perceiv- 
ing it. 

Theaet. Of course. 

Socr. Then my perception is true for me ; for its object 
at any moment is my reality, 3 and I am, as Protagoras 
says, a judge of what is for me, that it is, and of what 
is not, that it is not. 
Theaet. So it appears. 

d. Socr. If, then, I am infallible and make no mistake in 
my state of mind about what is or becomes, how can I 

1 Without the co-operation of an object of which I am percipient, as the 
next speech explains, toiovtos = ovtcds aloBavo^evos (159E, 7-8). 

1 i.e. neither subject nor object can produce just that sensation and quality 
in conjunction with any other object or subject ; and neither of the two can 
produce offspring ' for itself ' without the other. 

8 rrjs cpijs ovalas = rcbv ifxoi ovtcov, what is real for me. Socrates is here 
stating the claim that perception is true as having the real (to ov) for its 
object, as well as its claim to infallibility, next mentioned. The weak point 
is that ' my reality ' is in fact only ' what becomes for me \ not genuinely 
real in Plato's sense. Note that in his next speech Socrates speaks of what 
' is or becomes '. 


THEAETETUS 1 60E-161 b 

i6od. fail to have knowledge of the things of which I have per- 
ception ? 

Theaet. You cannot possibly fail. 

Socr. So you were perfectly right in saying that knowledge 
is nothing but perception ; and it has turned out that these 
three doctrines coincide : the doctrine of Homer and 
Heracleitus and all their tribe that all things move like 
flowing streams ; the doctrine of Protagoras, wisest of 
men, that Man is the measure of all things ; and Theaetetus' 
E. conclusion that, on these grounds, it results that perception 
is knowledge. 

Is it not so, Theaet etus ? May we say that this is your 
newborn child which I have brought to birth ? What do 
you say ? 
Theaet. I can only agree, Socrates. 

Thus Socrates claims to have brought to light the full meaning 
of Theaetetus' identification of knowledge with perception. The 
first step was to analyse the nature of perception. Plato was 
forced to give his own account of the process, based on the Hera- 
cleitean principle which he accepted so far as sensible things are 
concerned. He has also adopted Protagoras' doctrine as applied 
to my immediate awareness of sense-objects, including dream 
images and hallucinations. In this field I am the measure of 
what ' becomes for me ' or ' appears to me ' ; if wine tastes sour 
to me, no one can say I am mistaken because the wine really is 
sweet in itself. So perception has one of the two marks of know- 
ledge, infallibility. And, if we can accept Protagoras' identification 
of what appears to me with what is, or is real, ignoring the addition 
1 for me ' or ' to me ' and the distinction between being and becom- 
ing, the case will be complete. Socrates has, at any rate, dealt 
fairly with Theaetetus in making the best case for his candidate 
that can be made. 

160E-161B. Interlude. Criticism begins 

A short interlude marks that the first stage of the dialectical 
process is now complete. Socrates has drawn out the full mean- 
ing of Theaetetus' suggested definition of knowledge. The second 
stage, criticism, is now to begin. What follows has sometimes 
been misunderstood through a failure to see what the scope of the 
criticism precisely is. 

First, it is not directed against the theory of perception as a 
whole, or against those elements in the theory which Plato has 
adopted from Heracleitus and Protagoras. If the account of the 
nature of perception were now to be rejected, obviously we should 



not know what we were denying when we finally deny that per- 
ception is knowledge. This fabric stands unshaken. The process 
of perception is such as it has been described. The question is 
whether, being such, it possesses all the marks of knowledge. 

At the same time, Plato has to explain exactly how much he 
has taken from Heracleitus and Protagoras, and exactly where 
he refuses to follow them further. The Heracleitean dogma ' All 
things are in motion ' can be accepted if ' all things ' is restricted 
(as it is in the theory of perception) to sensible physical objects. 
But there are other things — intelligible objects — to which it does 
not apply ; and these are, for Plato, the true realities. If these 
were always changing, no true statement could ever be made and 
there could be no such thing as knowledge or discourse. Similarly, 
the Protagorean maxim, man the measure of all things, can be 
accepted if ' all things ' is restricted (as our theory restricts it) to 
the immediate objects of our awareness in sensation or perception, 
in which no element of judgment is supposed to be involved. But 
Protagoras' phrase ' what appears to me ' was not so restricted ; 
it included what appears true to me, what I judge or think or 
believe to be true. Plato will deny that whatever I judge to be 
true must be true, simply, or even true to me or for me. Hence, 
in the following argument, criticism is directed partly against the 
claim of perception, as Plato has denned it, to be knowledge ; 
partly against those elements of Heracleitean and Protagorean 
doctrine which go beyond what Plato has accepted. 

i6oe. Socr. Here at last, then, after our somewhat painful 
labour, is the child we have brought to birth, whatever sort 
of creature it may be. His birth should be followed by the 
ceremony of carrying him round the hearth x ; we must 
look at our offspring from every angle to make sure we 

161. are not taken in by a lifeless phantom not worth the rear- 
ing. Or do you think an infant of yours must be reared 
in any case and not exposed ? Will you bear to see him 
put to the proof, and not be in a passion if your first-born 
should be taken away ? 

Theod. Theaetetus will bear it, Socrates ; he is thor- 
oughly good-tempered. But do explain what is wrong with 
the conclusion. 

Socr. You have an absolute passion for discussion, 
Theodorus. I like the way you take me for a sort of bag 

1 The Amphidromia was held a few days after birth. The infant received 
its name and was associated with the family cult by being carried round the 
central hearth. 


THEAETETUS 161b-163a 

161. full of arguments, and imagine I can easily pull out a proof 

b. to show that our conclusion is wrong. You don't see what 
is happening : the arguments never come out of me, they 
always come from the person I am talking with. I am 
only at a slight advantage in having the skill to get some 
account of the matter from another's wisdom and entertain 
it with fair treatment. So now, I shall not give any explana- 
tion myself, but try to get it out of our friend. 

Theod. That is better, Socrates ; do as you say. 

161B-163A. Some objections against Protagoras 

Theodorus is here drawn into the discussion, to mark that the 
first objections will be made against his personal friend, Protagoras. 

161 b. Socr. Well then, Theodorus, shall I tell you a thing that 
surprises me in your friend Protagoras ? 

c. Theod, What is that ? 

Socr. The opening words of his treatise. In general, I 
am delighted with his statement that what seems to any- 
one also is ; but I am surprised that he did not begin his 
Truth with the words : The measure of all things is the 
pig, or the baboon, or some sentient creature still more 
uncouth. There would have been something magnificent 
in so disdainful an opening, telling us that all the time, 
while we were admiring him for a wisdom more than mortal, 

d. he was in fact no wiser than a tadpole, to say nothing of 
any other human being. What else can we say, Theodorus ? 
If what every man believes as a result of perception is 
indeed to be true for him ; if, just as no one is to be a 
better judge of what another experiences, so no one is 
better entitled to consider whether what another thinks 
is true or false, and (as we have said more than once) every 
man is to have his own beliefs for himself alone and they are 
all right and true — then, my friend, where is the wisdom of 

E. Protagoras, to justify his setting up to teach others and to 
be handsomely paid for it, and where is our comparative 
ignorance or the need for us to go and sit at his feet, when 
each of us is himself the measure of his own wisdom ? 
Must we not suppose that Protagoras speaks in this way to 
flatter the ears of the public ? I say nothing of my own 
case or of the ludicrous predicament to which my art of 
midwifery is brought, and, for that matter, this whole busi- 
ness of philosophic conversation ; for to set about over- 
hauling and testing one another's notions and opinions when 



162. those of each and every one are right, is a tedious and mon- 
strous display of folly, if the Truth of Protagoras is really 
truthful and not amusing herself with oracles delivered 
from the unapproachable shrine of his book. 
Theod. Protagoras was my friend, Socrates, as you were 
saying, and I would rather he were not refuted by means 
of any admissions of mine. On the other hand, I cannot 
resist you against my convictions ; so you had better go 
back to Theaetetus, whose answers have shown, in any case, 
how well he can follow your meaning. 

b. SocR. If you went to a wrestling-school at Sparta, Theo- 
dorus, would you expect to look on at the naked wrestlers, 
some of them making a poor show, and not strip so as to 
let them compare your own figure ? 
Theod. Why not, if they were likely to listen to me 
and not insist, just as I believe I shall persuade you to 
let me look on now ? The limbs are stiff at my age ; and 
instead of dragging me into your exercises, you will try 
a fall with a more supple youth. 

Socr. Well, Theodorus, as the proverb says, ' what likes 
you mislikes not me/ So I will have recourse to the 

C wisdom of Theaetetus. 

Tell me, then, first, Theaetetus, about the point we have 
just made : are not you surprised that you should turn 
out, all of a sudden, to be every bit as wise as any other 
man and even as any god ? Or would you say that Protag- 
oras' maxim about the measure does not apply to gods 
just as much as to men ? 

Theaet. Certainly I think it does ; and, to answer your 
question, I am very much surprised. When we were dis- 

d. cussing what they mean by saying that what seems to 
anyone really is to him who thinks it so, 1 that appeared 
to me quite satisfactory ; but now, all in a moment, it has 
taken on a very different complexion. 
Socr. That, my friend, is because you are young ; so you 
lend a ready ear to clap-trap and it convinces you. Protag- 
oras or his representative will have an answer to this. 
He will say : ' You good people sitting there, boys and 
old men together, this is all clap-trap. You drag in the 
gods, whose existence or non-existence I expressly refuse 

E. to discuss in my speeches and writings, and you count 

1 The ambiguity of hoKeiv, including ' what seems ' (to Bokovv), which might 
mean only perception, and ' he who thinks ' or ' fudges ' (oSokwv), is here neatly 


THEAETETUS 163a-164b 

162E. upon appeals to the vulgar such as this : how strange that 
any human individual is to be no wiser than the lowest 
of the brutes ! You go entirely by what looks probable, 
without a word of argument or proof. If a mathematician 
like Theodorus elected to argue from probability in geometry, 
he wouldn't be worth an ace. So you and Theodorus might 
consider whether you are going to allow questions of this 

163. importance to be settled by plausible appeals to mere 

Theaet. Well, you would not think that right, Socrates, 
any more than we should. 

Socr. It seems, then, we must attack the question in 
another way. That is what you and Theodorus think. 
Theaet. Certainly we must. 

Socrates has brought against Protagoras two objections, which 
are not of equal cogency. (1) Why not ' Pig the measure of all 
things ' ? On the level of mere sensation, man has no privileged 
position. The pig, or the anthropomorphic god (if such a being 
exists), is just as much the measure of his own sensations. Plato, 
who confined his acceptance of the maxim to that level, would 
admit this. But Protagoras went beyond sensation and per- 
ception to include under ' what seems to me ' what I think or 
judge to be true. The serious objection is : (2) ' If what every 
man believes as the result of perception is to be true for him ', 
how can any man be wiser than another ? Here Plato parts 
company with Protagoras. When we return to these objections, 
we shall deny that every man is the measure of the truth of his 
^ own judgments. 

163A-164B. Objections to a simple identification of Perceiving and 

Meanwhile, Protagoras having registered his protest against 
clap-trap, the question of judgment is dropped. Socrates turns 
to some preliminary criticisms of Theaetetus' proposition : Per- 
ception is knowledge. These criticisms are made here because 
Protagoras will be able to answer them presently in his Defence. 
They take ' perception ', as we have now analysed it, in the strictest 
and narrowest sense, and point out that we shall find ourselves in 
curious difficulties if we assert that such perception is the only 
form of knowledge. The objections are later called captious or 
' eristic ', not because they are invalid, but because they take 
Theaetetus' statement more literally than he intended. They 
serve a purpose by calling atteirtion to various meanings of the 



word 'know' (emaraadai). (i) I am said to 'know* Syriac l 
when I understand the meaning of written or spoken symbols. 
(2) I ' know ' Socrates when I have become acquainted with a certain 
person by sense-perception and possess a record of this acquaintance 
in memory. In neither of these senses can ' I know ' be simply 
equated with ' I am perceiving \ It is necessary and fair to make 
Theaetetus see what a simple identification of perceiving and 
' knowing ' commits him to. 

163A. Socr. Let us look at it in this way, then — this question 
whether knowledge and perception are, after all, the same 
thing or not. For that, you remember, was the point to 
which our whole discussion was directed, and it was for 
its sake that we stirred up all this swarm of queer doctrines, 
wasn't it ? 
Theaet. Quite true. 

b. Socr. Well, are we going to agree that, whenever we 
perceive something by sight or hearing, we also at the same 
time know it ? Take the case of a foreign language we 
have not learnt. Are we to say that we do not hear the 
sounds that foreigners utter, or that we both hear and 
know what they are saying ? Or again, when we don't 
know our letters, are we to maintain that we don't see 
them when we look at them, or that, since we see them, 
we do know them ? 

Theaet. We shall say, Socrates, that we know just so 
much of them as we do see or hear. The shape and colour 
of the letters we both see and know ; we hear and at the 

c. same time know the rising and falling accents of the voice ; 
but we neither perceive by sight and hearing nor yet know 
what a schoolmaster or an interpreter could tell us about 

Socr. Well done, Theaetetus. I had better not raise 
objections to that, for fear of checking your growth. 2 But 
look, here is another objection threatening. How are we 
going to parry it ? 
Theaet. What is that ? 

d. Socr. It is this. Suppose someone to ask : ' Is it possible 

1 ZvpiarX itriaraadai (Xenophon), ypafifxara €7TicrTa<r0ai. 

* Socrates might object that to ' know ' a language does not mean hearing 
unintelligible sounds or seeing black marks on paper, but to know the meaning, 
which we do not see or hear. But Plato does not want to embark on a dis- 
cussion of what it is we know when we know the meaning of words. That 
would involve bringing in the Forms, which he is determined, so far as possible, 
to leave out of account. So the point is not pressed. 


THEAETETUS 163a-164b 

163D. for a man who has once come to know something and 
still preserves a memory of it, not to know just that thing 
that he remembers at the moment when he remembers 
it ? ' This is, perhaps, rather a long-winded way of putting 
the question. I mean : Can a man who has become ac- 
quainted x with something and remembers it, not know it ? 
Theaet. Of course not, Socrates ; the supposition is 

Socr. Perhaps I am talking nonsense, then. But con- 
sider : you call seeing ' perceiving ', and sight ' perception ', 
don't you ? 
Theaet. I do. 
e. Socr. Then, according to our earlier statement, 2 a man 
who sees something acquires from that moment knowledge 
of the thing he sees ? 
Theaet. Yes. 

Socr. Again, you recognise such a thing as memory ? 
Theaet. Yes. 

Socr. Memory of nothing, or of something ? 
Theaet. Of something, surely. 

Socr. Of what one has become acquainted with and per- 
ceived — that sort of things ? 
Theaet. Of course. 

Socr. So a man sometimes remembers what he has seen ? 
Theaet. He does. 

Socr. Even when he shuts his eyes ? Or does he forget 
when he shuts them ? 

Theaet. No, Socrates ; that would be a monstrous thing 
to say. 

164. Socr. All the same, we shall have to say it, if we are to 
save our former statement. Otherwise, it goes by the 

Theaet. I certainly have a suspicion that you are right, 
but I don't quite see how. You must tell me. 
Socr. In this way. One who sees, we say, acquires know- 
ledge of what he sees, because it is agreed that sight or 
perception and knowledge are the same thing. 
Theaet. Certainly. 

Socr. But suppose this man who sees and acquires know- 
ledge of what he has seen, shuts his eyes ; then he remembers 
the thing, but does not see it. Isn't that so ? 

1 fiavddveiv here is wider than ' learn ', and equivalent to the phrase ' come 
to know something ' (cVtaT^cuv yeveodai) above. 

2 The simple identification of perceiving with knowing, recalled at 163A. 



164. Theaet. Yes. 

b. Socr. But ' does not see it ' means ' does not know it ', 
since ' sees ' and ' knows ' mean the same. 
Theaet. True. 

Socr. Then the conclusion is that a man who has come 
to know a thing and still remembers it does not know it, 
since he does not see it ; and we said that would be a 
monstrous conclusion. 
Theaet. Quite true. 

Socr. Apparently, then, if you say that knowledge and 
perception are the same thing, it leads to an impossibility. 
Theaet. So it seems. 

Socr. Then we shall have to say they are different. 
Theaet. I suppose so. 

In this argument memory first comes into sight. Remembering 
is a kind of knowing different from perceiving as we have analysed 
it. We seem to have immediate awareness of past objects not now 
given in the actual process of perception. If Theaetetus* definition 
of knowledge as perception is to be saved, ' perception ' must be 
stretched to cover awareness of memory-objects. Since there would 
be no objection to that, Socrates here breaks off what threatens 
to become a mere dispute about words. The conclusion stands, 
however, that ' I know ' has other meanings than ' I am (now) 
perceiving \ And the nature of memory will call for analysis later. 

164C-165E. Socrates undertakes to defend Protagoras 

In an interlude Socrates consents to state, on Protagoras' behalf, 
a reply to the criticism urged against Man the measure of all things. 
Incidentally, he adds another ' eristic ' objection to Theaet etus* 
equation of perceiving with knowing. 

164c Socr. What, then, can knowledge be ? Apparently we 
must begin all over again. But wait a moment, Theaetetus. 
What are we doing ? 
Theaet. Doing about what ? 

Socr. It seems to me we are behaving towards our theory 
like an ill-bred gamecock who springs away from his adver- 
sary and starts crowing over him before he is beaten. 
Theaet. How so ? 

Socr. It looks as if we were content to have reached an 
agreement resting on mere verbal consistency and to have 
got the better of the theory by the methods of a professional 
controversialist. We profess to be seeking wisdom, not 


THEAETETUS 164c-165e 

164D. competing for victory, but we are unconsciously behav- 
ing just like one of those redoubtable disputants. 
Theaet. I still don't understand what you mean. 
Socr. Well, I will try to make the point clear, so far as I 
can see it. We were asking whether one who had become 
acquainted with something and remembered it could fail to 
know it. Then we pointed out that a man who shuts his 
eyes after seeing something, remembers but does not see ; 
and so concluded that at the same moment he both remem- 
bers the thing and does not know it. That, we said, was 
impossible. And so no one was left to tell Protagoras' 
tale \ or yours either, about knowledge and perception 
being the same thing. 
e. Theaet. So it appears. 

Socr. I fancy it would be very different if the author of 
the first story were still alive. He would have put up a 
good fight for his offspring. But he is dead, and here are 
we trampling on the orphan. Even its appointed guardians, 
like Theodorus here, will not come to the rescue. However, 
we will step into the breach ourselves and see that it has fair 
Theod. In point of fact, Socrates, it is rather Callias, son 

165. of Hipponicus, 2 who is Protagoras' trustee. My own inclina- 
tions diverted me at rather an early age from abstract dis- 
cussions to geometry. All the same, I shall be grateful for 
any succour you can give him. 

Socr. Very good, Theodorus. You shall see what my help 
will amount to. For one might commit oneself to even 
stranger conclusions, if one were as careless in the use of 
language as we commonly are in our assertions and denials. 
Am I to enlarge upon this to you or to Theaet etus ? 
Theod. To the company in general ; but let the younger 
man answer your questions. It will not be such a disgrace 
b. to him to be caught tripping. 

Socr. Let me put, then, the most formidable poser of all, 
which I take to be this : Can the same person know some- 
thing and also not know that which he knows ? 
Theod. Well, Theaet etus, what are we to answer ? 
Theaet. That it is impossible, I suppose. 
Socr. Not if you say that seeing is knowing. How are 
you going to deal with a question that leaves no loophole, 

1 A proverbial expression. 

2 A wealthy amateur of sophistry, who had entertained Protagoras on his 
visit to Athens. 



165 b. when you are trapped like a beast in a pit and an imperturb- 
able gentleman puts his hand over one of your eyes and asks 

c. if you can see his coat with the eye that is covered ? 
Theaet. I suppose I should say : No, not with that one, 
but I can with the other. 

Socr. So you both see and do not see the same thing at 
the same time ? 

Theaet. Yes, in a sort of way. 

Socr. Never mind about the sort of way, he will reply ; 
that was not the question I set you, but whether, when 
you know a thing, you also do not know it. In this instance 
you are obviously seeing something you don't see, and you 
have agreed that seeing is knowing and not seeing is not 
knowing. Now draw your conclusion. What is the con- 
sequence ? 

d. Theaet. Well, I conclude that the consequence contradicts 
my thesis. 

Socr. Yes, and you might have been reduced to the same 
condition by a number of further questions : whether know- 
ing can be keen or dim ; whether you can know from close 
at hand what you cannot know from a distance, or know the 
same thing with more or less intensity. A mercenary skir- 
misher in the war of words might lie in wait for you armed 
with a thousand such questions, once you have identified 
knowledge and perception. He would make his assaults 
upon hearing and smelling and suchlike senses and put you 

e. to confusion, sustaining his attack until your admiration 
of his inestimable skill betrayed you into his toils ; and 
thereupon, leading you captive and bound, he would hold 
you to ransom for such a sum as you and he might agree 
upon. 1 

And now, perhaps, you may wonder what argument 
Protagoras will find to defend his position. Shall we try 
to put it into words ? 
Theaet. By all means. 

The ' most formidable ' objection here added is, like the earlier 
ones, valid against Theaetetus' position, since he has accepted the 
account of perception as the commerce between a sense-organ and 
an external object. If that is what perception is, then to identify 
it with knowledge does lead to these absurdities. The objections 

1 Protagoras, if a pupil objected to the fee he charged, made him swear 
in a temple how much he thought what he had learnt was worth. Protag. 
328B ; Ar., E.N. 1164a, 24. 


THEAETETUS 165e-168c 

do not touch Protagoras, who did not limit knowledge to perception. 
They are called captious because they only apply to Theaetetus' 
statement when that is taken more literally than he meant, and do 
not apply to Protagoras, upon whom Socrates has seemed to father 
all this complex of doctrines he has constructed by his dialectical 
combinations. Such cavils do not dispose of the whole point of 
view which Theaetetus meant to bring forward, and we do not want 
to quarrel about words. Further, they do not impair Plato's own 
doctrine of the nature of sense-perception, or shake the claim of 
perception, as so defined, to yield infallible awareness of a private 
object, an element in that doctrine borrowed from Protagoras himself. 
It still remains to be shown why Plato refuses to call such awareness 
' knowledge \ Accordingly, he admits frankly that the whole posi- 
tion has not been disposed of by means of a few essays in sophistical 

165E-168C. The Defence of Protagoras 

The Defence now put by Socrates into the mouth of Protagoras 
falls into three main divisions. First comes a protest against the 
' captious ' objections and a reply to them. The central and most 
important part attempts to meet the really damaging criticism of 
Protagoras himself : If every man is the measure of his own judg- 
ments, how can Protagoras set up to be wiser than others ? Finally, 
in a peroration, the sophist is (ironically) represented as exhorting 
the dialectician to argue seriously, not catching at words, but trying 
to understand what the opponent really means. 

Socr. No doubt, then, Protagoras will make all the points 
we have put forward in our attempt to defend him, and 
166. at the same time will come to close quarters with the assail- 
ant, dismissing us with contempt. 1 ' Your admirable Soc- 
rates ', he will say, ' finds a little boy who is scared at being 
asked whether one and the same person can remember and 
at the same time not know one and the same thing. When 
the child is frightened into saying No, because he cannot 
foresee the consequence, Socrates turns the conversation so 
as to make a figure of fun of my unfortunate self. You take 
things much too easily, Socrates. The truth of the matter 
is this : when you ask someone questions in order to canvass 
some opinion of mine and he is found tripping, then I am 

1 Protagoras will both (re) urge, as we have done for him, that we are 
talking clap-trap (162D), that verbal disputation is futile (164B) and we must 
use words more carefully (165A), and (koI) will come to grips (not with us, but) 
with the sophistic skirmisher and his armoury of eristic cavils, despising us 
for our feeble surrender to such weapons. 



166. refuted only if his answers are such as I should have given ; 
B. if they are different, it is he who is refuted, not I. For 
instance, do you think you will find anyone to admit that 
one's present memory of a past impression is an impression 
of the same character as one had during the original experi- 
ence, which is now over ? It is nothing of the sort. Or 
again, will anyone shrink from admitting that it is possible 
for the same person to know and not to know the same thing ? 
Or, if he is N frightened of saying that, will he ever allow that 
a person who is changed is the same as he was before the 
change occurred ; or rather, that he is one person at all, 
and not several, indeed an infinite succession of persons, 
c. provided change goes on happening — if we are really to be 
on the watch against one another's attempts to catch at 
words ? 

Protagoras here makes three replies : (i) The first is to the objec- 
tion (163D) : You admit I can remember and so ' know ' an object 
I am not now seeing ; but you say ' I do not see ' = ' I do not 
know ' ; therefore I do not know what I remember, and we have 
the contradiction : I know and do not know the same thing. Protag- 
oras replies : The image before my memory is not the same thing 
as a present sense-impression or even like it. So it is not true that 
I know (remember) and do not know (see) the same thing. All 
that the objection in fact established was that ' perception ' must 
be stretched to include awareness of memory images. 

(2) ' No one will shrink from admitting that the same person can 
know and not know the same thing.' This replies to the ' most 
formidable ' puzzle of the man with one eye open, one shut (i66b). 
Theaetetus did suggest the answer : If we identify perception with 
the physical commerce between organ and object, one of my eyes 
does know the object, the other does not. This reply was brushed 
aside ; and if we shrink from it, Protagoras says, another answer 
is ready. 

(3) We have no right to speak of a person as the same at different 
moments. This reply is based on the theory of perception itself, 
which holds that the subject (organ) never remains the same for 
two moments together. Socrates himself has used this premiss for 
his conclusion at 160 a : No one can have the same perception twice. 
We have, in fact, spoken all through as if the physical organ were 
the subject that perceives, and the person a mere bundle of sense- 
organs. Hence we could argue that Socrates-well was ' totally 
different ', as a measure of the sweetness or sourness of wine, from 
Socrates-ill. If the subject, as well as the object, is perpetually 


THEAETETUS 165e-168c 

changing, objections which turn upon the same person knowing and 
not knowing the same thing fall to the ground. 

Thus the captious objections to Theaetetus' position are disposed 
of. We now turn to Protagoras' own doctrine and Socrates' criti- 
cisms of that (161c ff.). 

1 66c. Socr. (continues). ' No/ he will say ; ' show a more 
generous spirit by attacking what I actually say ; and prove, 
if you can, that we have not, each one of us, his peculiar 
perceptions, or that, granting them to be peculiar, it would 
not follow that what appears to each becomes — or is, if we 
may use the word ' is '—for him alone to whom it appears. 
With this talk of pigs and baboons, you are behaving like 
a pig yourself, 1 and, what is more, you tempt your hearers 
d. to treat my writings in the same way, which is not fair. 

So much for the objection : ' Why not pig the measure of all 
things ? ' That really needs no answer. For the rest, Socrates 
will not attempt to disprove the propositions here asserted : that 
each man has his private sensations and perceptions, which are 
infallible. This was precisely the Protagorean element adopted by 
Plato himself. Protagoras is not responsible for Theaetetus' sugges- 
tion, interpreted as asserting that knowledge consists solely of such 
perceptions. On the other hand, the doctrine ' man the measure ' 
was not confined to perception, but included judgment. To this 
extension it was objected : If each man is the measure of his own 
judgments or beliefs, how can one be wiser than another ? Here 
we come to the core of the Defence, which attempts to explain how 
one man can be wiser than another, although every man's judgments 
are true for him. 

i66d. Socr. (continues). ' For I do indeed assert that the truth 
is as I have written : each one of us is a measure of what is 
and of what is not ; but there is all the difference in the 
world between one man and another just in the very fact 
that what is and appears to one is different from what is 
and appears to the other. And as for wisdom and the 
wise man, I am very far from saying they do not exist. 
By a wise man I mean precisely a man who can change 
any one of us, when what is bad appears and is to him, and 
make what is good appear and be to him. In this statement, 
again, don't set off in chase of words, but let me explain 

1 The pig, in Greek, is an emblem of stupidity (a/ia0ux). Lack. 169D : ' Would 
not any pig know . . .' Cic., Ac. Post, i, 5, 18 : non sus docet Minervam. 
This remark is less offensive than the English sounds. 



i66e. still more clearly what I mean. Remember how it was put 
earlier in the conversation : to the sick man his food appears 
sour and is so ; to the healthy man it is and appears the 
opposite. Now there is no call to represent either of the 
two as wiser — that cannot be — nor is the sick man to be 

167. pronounced unwise because he thinks 1 as he does, or the 
healthy man wise because he thinks differently. What is 
wanted is a change to the opposite condition, because the 
other state is better. 

1 And so too in education a change has to be effected from 
the worse condition to the better ; only, whereas the 
physician produces a change by means of drugs, the sophist 
does it by discourse. It is not that a man makes someone 
who previously thought what is false think what is true 
(for it is not possible either to think the thing that is not 
or to think anything but what one experiences, and all 
B. experiences are true) ; rather, I should say, when someone 
by reason of a depraved condition of mind has thoughts of 
a like character, one makes him, by reason of a sound 
condition, think other and sound thoughts, which some 
people ignorantly call true, whereas I should say that one 
set of thoughts is better than the other, but not in any way 
truer. 2 And as for the wise, my dear Socrates, so far from 
calling them frogs, I call them, when they have to do with 
the body, physicians, and when they have to do with plants, 
husbandmen. For I assert that husbandmen too, when 
plants are sickly and have depraved sensations, substitute 
c. for these sensations that are sound and healthy 3 ; and 
moreover that wise and honest public speakers substitute 
in the community sound for unsound views of what is 

1 ' Thinks ', ' judges ' (Sofafci), here replaces ' appears ' {<f>alveoQai), What 
is meant is the judgment stating the fact of a sense-impression : ' This food 
seems and is to me sour.' If Socrates' earlier expression, ' what every man 
believes as the result of perception ' (o dv 8S alad^aecos $o£d£r), i6id) is restricted 
to such judgments, they are not ignorant or foolish judgments ; nor are they 

2 The text is doubtful. The best sense is obtained by taking rts (167A, 7) 
as the subject of a single sentence from eVcl (a, 6) to ot)8cV (b, 4). Read 
nov-qpa and xPV ar fi ( sc - ^"Xl* *&> w i*h W.) and omit to. <f>avrda^ara (withDiels, 
Vors* ii, 225). It is the sophist, not the xPl^rj e£is, that ' makes ' the change 
to sound thoughts. The reading xPV^r} will then be explained as an attempt 
to provide the inolrjae following it with a subject, made by someone who did 
not see that rts (governing the earlier i-rroC-qae, a. 7) is still the subject. 

3 Omitting re ko.1 dX-qdets. Diels' suggestion (Vors.* ii, 225) ware teal dXrjOels 
gives a wrong sense, for the unhealthy sensations are also true. The con- 
jectures dXrjdeias (Schleiermacher), egeis (Dies), nddas (Richards) are not con- 


THEAETETUS 165e-168c 

167c. right. For I hold that whatever practices seem right and 
laudable to any particular State are so, for that State, so 
long as it holds by them. Only, when the practices are, in 
any particular case, unsound for them, the wise man sub- 
stitutes others that are and appear sound. On the same 
principle the sophist, since he can in the same manner guide 
his pupils in the way they should go, is wise and worth a 
D. considerable fee to them when their education is com- 
pleted. In this way it is true both that some men are wiser 
than others and that no one thinks falsely ; and you, 
whether you like it or not, must put up with being a measure, 
since by these considerations my doctrine is saved from 
shipwreck. 1 

In this central section there is no reason to doubt that Socrates 
is doing what he professes to do — defending Protagoras' thesis as 
Protagoras, if he were alive, would himself have defended it. 2 
The form of the argument is necessarily adapted to the context ; 
but the contents are, in all probability, Protagorean. Protagoras 
was the first to claim the title of ' Sophist ', with its suggestion of a 
superior wisdom. 3 He must have reconciled this claim with his 
doctrine that all opinions are equally true, and can only have done 
so by arguing, as he does here, that some opinions are ' better ', 
though not truer, than others, and that his own business, as an edu- 
cator, was to substitute better opinions for worse. The analogy 
of the husbandman substituting sound and healthy sensations in 
plants is an archaic touch, 4 suggesting that Plato may be drawing 
on Protagoras' own writings. Protagoras' special profession was 
to educate men and make them good citizens ; and he taught the 
art of Rhetoric, which was to enable the public speaker to offer 
good counsel to the assembly in an effective form. He must have 
held the corresponding view, here stated, about the laws and cus- 
toms of States, considered as the judgments or decisions (dotjai) of 
the community. Such laws and customs are ' right ' for that 
community so long as it holds by them ; but a wise statesman can 
try to substitute others that are ' better ' or ' sounder '. We may 
conclude that Plato here is fairly reproducing the standpoint of the 
historic Protagoras. 

1 aco^erat 6 X6yos seems to allude to nvOos aircoX^ro 6 IJpojTayopeios, 164D. Cf. 
Rep. 62 IB fivdos €0(1)07} koX ovk airibXero. 

2 Cf . H. Gomperz, Sophistik u. Rhetorik, p. 261. 3 Protag. 317B. 

4 Ps.-Arist., de plantis 815a, 15, attributes to Anaxagoras and Empedocjes 
the view that plants have sensation and pleasure and pain. The analogy 
between physician, husbandman, and educator recurs at Protag. 334, and 
Symp. 187A, both places where Plato is using earlier material. 



What, then, does the Defence actually maintain ? The argument 
advances, by stages, from the position where Plato has already 
agreed with Protagoras to the position which he will challenge in 
the sequel. 

(i) At the level of physical sensations or perceptions, it has been 
admitted (159-160) that a sick man's abnormal sensations are not 
less ' true ' than the healthy man's normal ones, and that they are 
partly determined by his own state of body. The physician, 
Protagoras argues, is called in to change that state, because it is 
generally agreed, by physicians and patients alike, that the healthy 
sensations are ' better \ ' Better ' presumably means ' more 
pleasant ' ; and each man is the sole judge of what he finds pleasant. 
The physician can be called ' wise ' because he knows how to change 
the worse state to a better. The point that remains obscure is 
what sort of knowledge enables him to do this. 

(2) The position of the educator is said to be analogous to the 
physician's ; it is his business to change our mental condition from 
unsound to sound, so that our judgments, beliefs, opinions, may be 
sounder, though not truer. The crucial statement is : ' It is not 
that a man makes someone who previously thought what is false 
think what is true ; for it is not possible either to think the thing 
that is not or to think anything beyond what one experiences, 
and all experiences are true.' The last words refer to Socrates' 
objection : ' If what every man believes as a result of perception 
is indeed to be true for him ; if, just as no one is to be a better judge 
of what another experiences (TzjdOog), so no one is better entitled 
to consider whether what another thinks (dd^av) is true or false ', 
where is the superior wisdom of Protagoras ? Protagoras' reply, 
' No one can think anything beyond what he experiences, and all 
experiences are true ', refers primarily to judgments which are 
supposed merely to register the fact of a present sensation : I 
judge that this wine seems sour to me. No one can challenge the 
truth of such a judgment. But in the same breath Protagoras 
extends this claim to all judgments or beliefs in the general state- 
ment : ' It is impossible to think the thing that is not ', i.e., to 
think what is false. The educator cannot, therefore, substitute 
truer beliefs ; but only * sounder ' ones. What ' sounder ' means is 
left obscure. It does not mean ' normal ', for that would set up the 
majority as a norm or measure for the minority. It can only mean 
more useful or expedient : a sound belief is one that will produce 
better effects in the future. 1 ' Better effects ', again, must mean 
effects that will seem better to me when the sophist has trained me. 

1 Protagoras' position should not be confused with modern Pragmatism, 
which does not assert that all beliefs must be equally true. 


THEAETETUS 165e-168c 

I shall then prefer my new beliefs to those which I now prefer. 
The s&nfcej argument applies to the laws and customs of the State. 
'Whatever practice seem right {dixaia) and laudable (xaM) to 
any particular State are so, for that State, so long as it holds by 
them \ Thus it is legally right and socially approved that Moham- 
medans should have several wives, Englishmen one only. But a 
statesman may try to substitute ' sounder ' customs. This again 
can only mean ' more expedient ' : an Englishman persuading 
Turks to adopt monogamy can only urge that the results will: seem 
better to the converted Turk. 

Such is Protagoras' position. The Defence now ends with a 
peroration, in which Protagoras lectures Socrates for frivolity and 
the points outstanding for serious criticism are recalled. 

167D. Socr. {continues), ' Now if you can dispute this doctrine in 
principle, do so by argument stating the case on the other 
side, or by asking questions, if you prefer that method, 
which has no terrors for a man of sense ; on the contrary 
it ought to be specially agreeable to him. Only there is 

e. this rule to be observed : do not conduct your questioning 
unfairly. It is very unreasonable that one who professes 
a concern for virtue should be constantly guilty of unfair- 
ness in argument. Unfairness here consists in not observing 
the distinction between a debate and a conversation. A 
debate need not be taken seriously and one may trip up an 
opponent to the best of one's power ; but a conversation 
should be taken in earnest ; one should help out the other 
party and bring home to him only those slips and fallacies 
168. that are due to himself or to his earlier instructors. If 
you follow this rule, your associates will lay the blame for 
their confusions and perplexities on themselves and not on 
you ; they will like you and court your society, and dis- 
gusted with themselves, will turn to philosophy, hoping to 
escape from their former selves and become different men. 
But if, like so many, you take the opposite course, you will 
reach the opposite result : instead of turning your com- 

B. panions to philosophy, you will make them hate the whole 
business when they get older. So, if you will take my 
advice, you will meet us in the candid spirit I spoke of, 
without hostility or contentiousness, and honestly consider 
what we mean when we say that all things are in motion 
and that what seems also is, to any individual or com- 
munity. The further question whether knowledge is, or 
is not, the same thing as perception, you will consider as a 



i68b. consequence of these principles, not (as you did just now) 

c. basing your argument on the common use of words and 
phrases, which the vulgar twist into any sense they please 
and so perplex one another in all sorts of ways.' 

So the Defence ends. The central part was confined to genuine 
Protagorean doctrine ; but here we are reminded that Socrates' 
dialectical construction has included also the Heracleitean flux and 
Theaetetus' claim that perception is the same thing as knowledge. 
All three elements still await serious criticism, and they are dealt 
with separately in the sequel, (i) The Protagorean thesis — Every 
judgment true for him who makes it — is refuted for the individual 
(169D-171C) and for the State (1770179B) ; next (2) the unrestricted 
doctrine — All things are in motion — is denounced as fatal to all 
discourse (179C-183B) ; and (3) the identification of perception 
with knowledge is Anally rejected J184B-186E). 

1680-169D. Interlude 

In an interlude Theodorus is again dra/wn into the discussion. 
This marks that the next section of the argument is directed against 
his friend_Prj3ta^oxas, who is gat^ieWregponsible for the two other 

168c. Socr. {continues). Such, Theodorus, is my contribution to 
the defence of your friend — the best I can make from my 
small means. Were he alive to speak for himself, it would 
be a much more impressive affair. 

Theod. You are not serious, Socrates ; your defence was 
most spirited. 

Socr. Thank you, my friend. And now, did you notice 
how Protagoras was reproaching us for taking a child to 

d. argue with and using the boy's timidity to get the better 
of his own position in what he called a mere play of wit, 
in contrast to the solemnity of his measure of all things, 
and how he exhorted us to be serious about his doctrine ? 
Theod. Of course I did, Socrates. 

Socr. What then ? Do you think we should do as he says ? 
Theod. Most certainly. 

Socr. Well, the company, as you see, are all children, 
except yourself. If we are to treat his doctrine seriously, 

e. as he enjoins, you and I must question one another. So 
we shall at any rate escape the charge of making light of it 
by discussing it with boys. 

Theod. Why, surely Theaetetus can follow up such an 


THEAETETUS 169d-171d 

i68e. investigation better than a great many men with long 

Socr. But not better than you, Theodoras. So don't 
imagine that you have no duty to your departed friend, but 
can leave it to me to make the best defence for him. Please 

169. come with us a little of the way at any rate — just until we 
know whether, in the matter of mathematical demonstrations, 
you cannot help being a measure, or everybody is just as 
competent as you in geometry and astronomy and all the 
other subjects you are supposed to excel in. 
Theod. It is no easy matter to escape questioning in your 
company, Socrates. I was deluded when I said you would 
leave me in peace and not force me into the ring like the 
Spartans : you seem to be as unrelenting as Skiron. The 
B. Spartans tell you to go away if you will not wrestle, but 
Antaeus is more in your line : you will let no one who comes 
near you go until you have stripped him by force for a trial 
of strength. 

Socr. Your comparisons exactly fit what is wrong with me, 
Theodoras ; but my capacity for endurance is even greater. 
I have encountered many heroes in debate, and times 
without number a Heracles or a Theseus has broken my head ; 
c. but I have so deep a passion for exercise of this sort that 
I stick to it all the same. So don't deny me the pleasure 
of a trial, for your own benefit as well as mine. 
Theod. I have no more to say ; lead me where you will. 
You are like Fate : no one can elude the toils of argument you 
spin for him. But I shall not be able to oblige you beyond 
the point you have proposed. 

Socr. Enough, if you will go so far. And please be on 
the watch for fear we should be betrayed into arguing 
D. frivolously and be blamed for that again. 
Theod. I will try as well as I can. 

169D-171D. Criticism of Protagoras' doctrine as extended to all 

Socrates now opens the attack on the genuinely Protagorean 
doctrine put forward in the central part of the Defence — the exten- 
sion of the maxim, Man the measure, beyond the field of immediate 
perception (where we accepted it) to all judgments. 

Our original objection (161 d) was : If all judgments are true to 
him who makes them, how can one man be wiser than another ? 
In the Defence Protagoras was represented as ' conceding ' that some 
are wiser than others, and this might seem to weaken his case. 



Socrates now observes that we ought to make sure of this step by 
deducing it formally from what Protagoras certainly did say, 
namely, that ' what seems to each man is to him \ Presumably, 
Plato wishes to avoid the imputation of attributing to Protagoras 
a statement which did not appear just in that form in his writings. 

169D. Socr. Let us begin, then, by coming to grips with the 
doctrine at the same point as before. Let us see whether 
or not our discontent was justified, when we criticised it 
as making every individual self-sufficient in wisdom. Pro- 
tagoras then conceded that some people were superior in 
the matter of what is better or worse, and these, he said, 
were wise. Didn't he ? 
Theod. Yes. 

Socr. If he were here himself to make that admission, 
e. instead of our conceding it for him in our defence, there 
would be no need to reopen the question and make sure of 
our ground ; but, as things are, we might be said to have 
no authority to make the admission on his behalf. So it 
will be more satisfactory to come to a more complete and 
clear agreement on this particular point ; for it makes a 
considerable difference, whether this is so or not. 
Theod. That is true. 

Socr. Let us, then, as briefly as possible, obtain his 
agreement, not through any third person, but from his 

170. own statement. 
Theod. How ? 

Socr. In this way. He says — doesn't he ? — that what 
seems true x to anyone is true for him to whom it seems so ? 
Theod. He does. 

Socr. Well now, Protagoras, we are expressing what seems 
true to a man, or rather to all men, when we say that 
everyone without exception holds that in some respects he 
is wiser than his neighbours and in others they are wiser 
than he. For instance, in moments of great danger and 
distress, whether in war or in sickness or at sea, men regard 
as a god anyone who can take control of the situation and 
b. look to him as a saviour, when his only point of superiority 
is his knowledge. Indeed, the world is full of people looking 
for those who can instruct and govern men and animals and 
direct their doings, and on the other hand of people who 
think themselves quite competent to undertake the teaching 

1 to ookovv here, as the context shows, mean ' what seems true '. Since 
Protagoras' maxim covered judgment, the interpretation is perfectly fair. 


THEAETETUS 169d-171d 

170B. and governing. In all these cases what can we say, if not 
that men do hold that wisdom and ignorance exist among 
them ? 

Theod. We must say that. 

Socr. And they hold that wisdom lies in thinking truly, 
and ignorance in false belief ? 

c. Theod. Of course. 

Socr. In that case, Protagoras, what are we to make of 
your doctrine ? Are we to say that what men think is 
always true, or that it is sometimes true and sometimes 
false ? From either supposition it results that their thoughts 
are not always true, but both true and false. For consider, 
Theodorus. Are you, or is any Protagorean, prepared to 
maintain that no one regards anyone else as ignorant or as 
making false judgments ? 
Theod. That is incredible, Socrates. 

d. Socr. That, however, is the inevitable consequence of the 
doctrine which makes man the measure of all things. 
Theod. How so ? 

Socr. When you have formed a judgment on some matter 
in your own mind and express an opinion about it to me, 
let us grant that, as Protagoras' theory says, it is true for 
you ; but are we to understand that it is impossible for us, 
the rest of the company, to pronounce any judgment upon 
your judgment ; or, if we can, that we always pronounce your 
opinion to be true ? Do you not rather find thousands of 
opponents who set their opinion against yours on every 
occasion and hold that your judgment and belief are 
false ? 
E. Theod. I should just think so, Socrates ; thousands and 
tens of thousands, as Homer says ; and they give me all 
the trouble in the world. 

Socr. And what then ? Would you have us say that in 
such a case the opinion you hold is true for yourself and 
false for these tens of thousands ? 
Theod. The doctrine certainly seems to imply that. 
Socr. And what is the consequence for Protagoras himself ? 
Is it not this : supposing that not even he believed in man 
being the measure and the world in general did not believe 
it either — as in fact it doesn't — then this Truth which he 
171. wrote would not be true for anyone ? If, on the other hand, 
he did believe it, but the mass of mankind does not agree 
with him, then, you see, it is more false than true by just 
so much as the unbelievers outnumber the believers. 



171. Theod. That follows, if its truth or falsity varies with 
each individual opinion. 

Socr. Yes, and besides that it involves a really exquisite 
conclusion. 1 Protagoras, for his part, admitting as he does 
that everybody's opinion is true, must acknowledge the 
truth of his opponents' belief about his own belief, where 
they think he is wrong. 
Theod. Certainly. 

b. Socr. That is to say, he would acknowledge his own belief 
to be false, if he admits that the belief of those who think 
him wrong is true ? 

Theod. Necessarily. 

Socr. But the others, on their side, do not admit to them- 
selves that they are wrong. 
Theod. No. 

Socr. Whereas Protagoras, once more, according to what 
he has written, admits that this opinion of theirs is as true 
as any other. 
Theod. Evidently. 

Socr. On all hands, then, Protagoras included, his opinion 
will be disputed, or rather Protagoras will join in the 
general consent — when he admits to an opponent the truth 

c. of his contrary opinion, from that moment Protagoras 
himself will be admitting that a dog or the man in the 
street is not a measure of anything whatever that he 
does not understand. Isn't that so ? 

Theod. Yes. 

Socr. Then, since it is disputed by everyone, the Truth 
of Protagoras is true to nobody — to himself no more than 
to anyone else. 

Theod. We are running my old friend too hard, Socrates. 
Socr. But it is not clear that we are outrunning the truth, 
my friend. Of course it is likely that, as an older man, he 
D. was wiser than we are ; and if at this moment he could pop 
his head up through the ground there as far as to the neck, 
very probably he would expose me thoroughly for talking 
such nonsense and you for agreeing to it, before he sank 
out of sight and took to his heels. However, we must do 
our best with such lights as we have and continue to say 
what we think. 

1 Sextus, Math, vii, 389, says that an argument of this form, known as 
' turning the tables ' (Trepirpo-nrj), was used against Protagoras by Democritus, 
as well as by Plato here. 


THE AETETUS 1 7 1 d- 1 72b 

Socrates' last words probably do not mean that Protagoras would, 
in Plato's opinion, have had any valid answer to make. The argu- 
ment has fairly deduced, on Protagoras' own principles, the conse- 
quences of asserting that what every man thinks true is true for 
him. It does follow for Protagoras' opponents that his doctrine 
is not true, and, for Protagoras himself, that their belief in its 
falsity is true for them. 

171D-172B. Restatement of the question : wherein lies the superiority 
of the wise ? 

This argument, however, is ad hominem. The real issue between 
Protagoras and Plato is too serious to be disposed of so lightly, 
and Socrates now gives the conversation a graver turn. He begins 
by restating the premiss on which all, including Protagoras, are 
agreed : that one man can be wiser than another. Wherein can 
such superiority lie ? Not in the field of immediate perception of 
sense-qualities : there (as Plato is careful to note once more) 
we have agreed with Protagoras that each man is the measure of 
what is, or rather ' becomes ', for him. But the Defence itself 
claimed a superiority in wisdom for the physician, the educator, 
and the statesman. All these undertake to change our condition 
and make ' better ' things ' appear and be ' to the individual or 
to the State. We have still to inquire what this profession implies. 

171D. Socr. {continues). Now, for instance, must we not say that 
everyone would agree at least to this : that one man can 
be wiser or more ignorant than another ? 
Theod. I certainly think so. 

Socr. And further, shall we say that the doctrine would 
find its firmest footing in the position we traced out in 
E. our defence of Protagoras : that most things — hot, dry, 
sweet, everything of that sort — are to each person as they 
appear to him ? Whereas, if there is any case in which the 
theory would concede that one man is superior to another, 
it might consent to admit that, in the matter of good or bad 
health, not any woman or child — or animal, for that matter 
— knows what is wholesome for it and is capable of curing 
itself ; but that here, if anywhere, one person is superior 
to another. 
Theod. I should certainly say so. 

172. Socr. And again in social matters, the theory will say 
that, so far as good and bad customs or rights and wrongs 
or matters of religion are concerned, whatever any State 
makes up its mind to enact as lawful for itself, really is 



172. lawful for it, and in this field no individual or State is wiser 
than another. But where it is a question of laying down 
what is for its advantage or disadvantage, once more there, 
if anywhere, the theory will admit a difference between 
two advisers or between the decisions of two different 
States in respect of truth, and would hardly venture to 
assert that any enactment which a State supposes to be 
B. for its advantage will quite certainly be so. 

The position taken up in the Defence is here restated fairly. 
The doctor has some wisdom or knowledge justifying his offer to 
change my condition to one in which things he calls ' better ' will 
appear and be to me. His case is parallel to that of the statesman, 
who uses his eloquence to recommend a change of custom or of 
law or a practical policy. If ' right ' means simply what is en- 
joined by law and a ' good custom ' one that is in fact socially 
approved, no State can claim to be wiser than another. But 
anyone who comes forward to recommend a change must claim 
that it will produce ' better ' results, that is to say, results which 
will appear as more advantageous when the change has been 
effected. When we return to this point later, it will be argued 
that the doctor's or the statesman's present judgment about what 
will be more advantageous in the future conflicts, ex hypothesi, 
with the judgment of his unconverted hearers, and that both cannot 
be true. This argument, however, is not developed until after 
the ' digression ', which now follows. 

172B-177C. Digression : the contrast of Philosophy and Rhetoric 

The occasion of this digression has not been well understood. 
Socrates breaks off at this point to suggest that some who ' do not 
argue altogether as Protagoras does ' may not accept the analogy 
that has just been drawn between the doctor's concern with the 
bodily health of the individual and the statesman's concern with 
questions of right and wrong. They will deny that ' right ' has 
any meaning at all other than what is publicly decreed at any 
time. This, as Socrates says, raises a larger issue than the argu- 

ent we were just embarking upon with Protagoras. 

72B. Socr. {continues). But, in that field I am speaking of — in 
right and wrong and matters of religion — people * are ready 
to affirm that none of these things is natural, with a reality 
of its own, but rather that the public decision becomes true 

1 The subject of the plural ideXovcri is not the same as the singular subject 
[o Xoyos) of the previous sentences, and accordingly not Protagoreans but (as 
Campbell says) ' certain persons who are presently denned \, 


THE AETETUS 1 72b- 1 77c 

172B. at the moment when it is made and remains true so long 
as the decision stands ; and those who do not argue alto- 
gether as Protagoras does carry on their philosophy on 
these lines. 1 
But one theory after another is coming upon us, Theo- 
c. dorus, and the last is more important than the one before. 

Editors have not seen clearly that this sentence does not amplify 
the preceding one, but introduces a new position held, not by 
Protagoras, but by people who do not state their position 
altogether as Protagoras stated his. Their view is the ' more 
important ' theory, involving larger issues than the restricted 
position we have just ascribed to Protagoras, the consideration of 
which is accordingly postponed. 

What is this larger theory ? Those who hold it are not ' incom- 
plete Protagoreans ', but go further than Protagoras himself. They 
deny the analogy between physical qualities (hot, dry, sweet, etc.) 
and moral qualities like ' just '. The hot and the cold, the dry 
and the moist, they will say, exist ' by nature ' ; and they would 
agree with Protagoras that the fact that one contrary appears to 
me, the other to you, is consistent with their having an objective 
being of their own. But ' just ' and ' unjust ', they say, have no 
status in Nature ; they are mere creations of convention or of the 
public decision of the community. We have no evidence that 
Protagoras went so far as this. 2 It is the extreme position formu- 
lated in the Republic by Thrasymachus, who denies that ' right ' 
has any natural validity : the word means nothing more than what 
the most powerful element in the State decrees for its own advan- 
tage (to rov xqeittovos GVjucpeQov). He would reject the distinction 
Socrates has just drawn between what is laid down as lawful and 
what is decided upon as advantageous (av/btcpegovra). When 
Socrates argued in the Republic (as he will later in the Theaetetus) 
that the strongest element in the State may be mistaken about 
its own advantage, Thrasymachus was not convinced. The 
atheists of Laws X (889 ff.) draw the same contrast between Nature 
and convention. Fire, Air, Water, and Earth exist by nature and 

1 Reading koX ocroi ye 817. . . \iyovai. "Oool av Xeyajot, would mean ' all who 
do not argue ', and we should then have to understand (with M. Dies and 
others) all who do not go so far as Protagoras. But these people go further. 
It is not true that everyone who stops short of Protagoras' position holds the 
extreme view here stated. 

2 His speech in the Protagoras 320 ff. recognises innate moral instincts of 
alScos and 81*77, existing in all men before society is formed. Education 
in virtue is a development of these natural instincts by a socialising process, 
making men good citizens of their own States. 



chance, without design ; and by the interplay of their active 
powers — hot, cold, dry, moist, etc. — produce the whole physical 
cosmos. But art or design arises only later ; it is mortal and of 
mortal origin. The whole of legislation, custom, and religion is 
' not by nature, but by art '. Conventions differ in different com- 
munities. * What is right (rd dlxoua) has no natural existence at 
all ; but men are perpetually disputing about it and altering it, 
and whatever alteration they make at any time is at that time 
authoritative, owing its existence to design and the laws, not in 
any way to nature ' (889E). This is precisely the position stated 
here, the extreme consequence of making man the measure of 
all things, but a consequence never ; so far as we know, drawn by 
Protagoras himself, who did not dream of subverting the basis of 

To Plato this thesis is the position of the arch-enemy ; the 
whole of the Republic is a reply to it. Here, acknowledging that 
it cannot be attributed to Protagoras, Socrates drops for a time 
the criticism of Protagoras* own theory, and replies indirectly in 
the ' digression ' that follows. A direct treatment would demand 
a repetition of the contents of the Republic and arguments sup- 
porting the Platonic thesis that the moral Forms, Justice and 
the rest, do ' exist by nature with a being of their own '. But the 
Forms are to be excluded, so far as possible, from this conversation, 
which discusses the claim of the world of appearances to yield 
knowledge without invoking the intelligible world. So Plato is 
content to indicate his answer by reviving the contrast drawn in 
the Gorgias and the Republic between the orator of the law court 
or the Assembly and the true statesman, the philosopher whose 
knowledge lies in that other realm of reality. The whole digression 
is studded with allusions to the Republic, and in the course of it 
the moral Forms are plainly, though unobtrusively, mentioned. 

172c. Theod. Well, Socrates, we have time at our disposal. 

Socr. Evidently. And it strikes me now, as often before, 
how natural it is that men who have spent much time in 
philosophical studies * should look ridiculous when they 
appear as speakers in a court of law. 
Theod. How do you mean ? 

Socr. When you compare men who have knocked about 
from their youth up in law courts and such places with 
others bred in philosophical pursuits, the one set seem to 
d. have been trained as slaves, the others as free men. 

1 0iXoao<f>ia has often a wide meaning covering all liberal studies (as at 143D) 
or ■ culture ' (as in Isocrates). 


THEAETETUS 172b-177c 

172D. Theod. In what way ? 

Socr. In the way you spoke of : the free man always 
has time at his disposal to converse in peace at his leisure. 
He will pass, as we are doing now, from one argument to 
another — we have just reached the third ; like us, he will 
leave the old for a fresh one which takes his fancy more ; 
and he does not care how long or short the discussion may 
be, if only it attains the truth. The orator is always talking 
E. against time, hurried on by the clock ; there is no space 
to enlarge upon any subject he chooses, but the adversary 
stands over him ready to recite a schedule of the points 
to which he must confine himself. He is a slave disputing 
about a fellow-slave before a master sitting in judgment 
with some definite plea in his hand ; and the issue is never 
indifferent, but his personal concerns are always at stake, 

173. sometimes even his life. Hence he acquires a tense and bitter 
shrewdness ; he knows how to flatter his master and earn his 
good graces, but his mind is narrow and crooked. An 
apprenticeship in slavery has dwarfed and twisted his growth 
and robbed him of his free spirit, driving him into devious 
ways, threatening him with fears and dangers which the 
tenderness of youth could not face with truth and honesty ; 
so, turning from the first to lies and the requital of wrong 

b. with wrong, warped and stunted, he passes from youth 
to manhood with no soundness in him and turns out, in 
the end, a man of formidable intellect — as he imagines. 

So much for the orator, Theodorus. Shall I now describe 
the philosophic quire to which we belong, or would you 
rather leave that and go back to our discussion ? We 
must not abuse that freedom we claimed of ranging from 
one subject to another. 
Theod. No, Socrates ; let us have your description first. 

c. As you said quite rightly, we are not the servants of the 
argument, which must stand and wait for the moment 
when we choose to pursue this or that topic to a conclusion. 
We are not in a court under the judge's eye, nor in the 
theatre with an audience to criticise our philosophic evolu- 

Socr. Then, if that is your wish, let us speak of the leaders 
in philosophy ; for the weaker members may be neglected. 
D. From their youth up they have never known the way to 
market-place or law court or council chamber or any other 
place of public assembly ; they never hear a decree read 
out or look at the text of a law ; to take any interest in 



173D. the rivalries of political cliques, in meetings, dinners, and 
merrymakings with flute-girls, never occurs to them even 
in dreams. Whether any fellow-citizen is well or ill born 
or has inherited some defect from his ancestors on either 
side, the philosopher knows no more than how many pints 
of water there are in the sea. He is not even aware that 
E. he knows nothing of all this ; for if he holds aloof, it is 
not for reputation's sake, but because it is really only his 
body that sojourns in his city, while his thought, disdaining 
all such things as worthless, takes wings, as Pindar says, 
' beyond the sky, beneath the earth ', searching the heavens 
and measuring the plains, everywhere seeking the true 

174. nature of everything as a whole, never sinking to what lies 
close at hand. 

Theod. What do you mean, Socrates ? 
Socr. The same thing as the story about the Thracian 
maidservant who exercised her wit at the expense of Thales, 
when he was looking up to study the stars and tumbled 
down a well. She scoffed at him for being so eager to 
know what was happening in the sky that he could not 
see what lay at his feet. Anyone who gives his life to philo- 
B. sophy is open to such mockery. It is true that he is unaware 
what his next-door neighbour is doing, hardly knows, 
indeed, whether the creature is a man at all ; he spends 
all his pains on the question, what man is, and what powers 
and properties distinguish such a nature from any other. 1 
You see what I mean, Theodorus ? 
Theod. Yes ; and it is true. 
Socr. And so, my friend, as I said at first, on a public 

c. occasion or in private company, in a law court or anywhere 
else, when he is forced to talk about what lies at his feet 
or is before his eyes, the whole rabble will join the maid- 
servants in laughing at him, as from inexperience he walks 
blindly and stumbles into every pitfall. His terrible 
clumsiness makes him seem so stupid. He cannot engage 
in an exchange of abuse, 2 for, never having made a study 
of anyone's peculiar weaknesses, he has no personal scandals 
to bring up ; so in his helplessness he looks a fool. When 

d. people vaunt their own or other men's merits, his unaffected 
laughter makes him conspicuous and they think he is 
frivolous. When a despot or king is eulogised, he fancies 

1 A clear allusion to the theory of Forms. The real object of knowledge 
is the Form ' Man ', not individual men. 

2 A constant feature of forensic speeches at Athens. 


THE AETETUS 1 72b-1 77c 

174D. he is hearing some keeper of swine or sheep or cows being 
congratulated on the quantity of milk he has squeezed 
out of his flock ; only he reflects that the animal that 
princes tend and milk is more given than sheep or cows 
to nurse a sullen grievance, and that a herdsman of this 
sort, penned up in his castle, is doomed by sheer press of 
E. work to be as rude and uncultivated as the shepherd in 
his mountain fold. He hears of the marvellous wealth of 
some landlord who owns ten thousand acres or more ; but 
that seems a small matter to one accustomed to think of 
the earth as a whole. When they harp upon birth — some 
gentleman who can point to seven generations of wealthy 
ancestors — he thinks that such commendation must come 
from men of purblind vision, too uneducated to keep their 

175. eyes fixed on the whole or to reflect that any man has 
had countless myriads of ancestors and among them any 
number of rich men and beggars, kings and slaves, Greeks 
and barbarians. To pride oneself on a catalogue of twenty- 
five progenitors going back to Heracles, son of Amphitryon, 
strikes him as showing a strange pettiness of outlook. He 
laughs at a man who cannot rid his mind of foolish vanity 

b. by reckoning that before Amphitryon there was a twenty- 
fifth ancestor, and before him a fiftieth, whose fortunes 
were as luck would have it. But in all these matters the 
world has the laugh of the philosopher, partly because he 
seems arrogant, partly because of his helpless ignorance 
in matters of daily life. 

Theod. Yes, Socrates, that is exactly what happens. 
Socr. On the other hand, my friend, when the philosopher 
drags the other upwards to a height at which he may 

c. consent to drop the question ' What injustice have I done 
to you or you to me ? ' and to think about justice and 
injustice in themselves, what each is, and how they differ 
from one another and from anything else l ; or to stop 
quoting poetry about the happiness of kings or of men with 
gold in store and think about the meaning of kingship and 
the whole question of human happiness and misery, what 
their nature is, and how humanity can gain the one and 
escape the other — in all this field, when that small, shrewd, 

d. legal mind has to render an account, then the situation is 
reversed. Now it is he who is dizzy from hanging at such 
an unaccustomed height and looking down from mid-air. 

1 The moral Forms are here openly mentioned, and there are allusions to 
the allegory of the Cave in Rep. vi. 



175D. Lost and dismayed and stammering, he will be laughed at, 
not by maidservants or the uneducated — they will not see 
what is happening — but by everyone whose breeding has 
been the antithesis of a slave's. 

Such are the two characters, Theodorus. The one is 
e. nursed in freedom and leisure, the philosopher, as you call 
him. He may be excused if he looks foolish or useless 
when faced with some menial task, if he cannot tie up bed- 
clothes into a neat bundle or flavour a dish with spices and 
a speech with flattery. The other is smart in the dispatch 
of all such services, but has not learnt to wear his cloak like 

176. a gentleman, or caught the accent of discourse that will 
rightly celebrate the true life of happiness for gods and 

Theod. If you could convince everyone, Socrates, as you 
convince me, there would be more peace and fewer evils in 
the world. 

Socr. Evils, Theodorus, can never be done away with, for 
the good must always have its contrary ; nor have they any 
place in the divine world ; but they must needs haunt this 
region of our mortal nature. That is why we should make 
all speed to take flight from this world to the other ; and 
B. that means becoming like the divine so far as we can, and 
that again is to become righteous with the help of wisdom. 
But it is no such easy matter to convince men that the 
reasons for avoiding wickedness and seeking after goodness 
are not those which the world gives. The right motive is 
not that one should seem innocent and good — that is no 
better, to my thinking, than an old wives' tale — but let us 
state the truth in this way. In the divine there is no 

c. shadow of unrighteousness, only the perfection of righteous- 
ness ; and nothing is more like the divine than any one of 
us who becomes as righteous as possible. It is here that a 
man shows his true spirit and power or lack of spirit and 
nothingness. For to know this is wisdom and excellence of 
the genuine sort ; not to know it is to be manifestly blind and 
base. All other forms of seeming power and intelligence 
in the rulers of society are as mean and vulgar as the 

d. mechanic's skill in handicraft. If a man's words and deeds 
are unrighteous and profane, he had best not persuade him- 
self that he is a great man because he sticks at nothing, 
glorying in his shame as such men do when they fancy that 
others say of them : They are no fools, no useless burdens 
to the earth, but men of the right sort to weather the storms 


THEAETETUS 172b-177c 

176D. of public life. Let the truth be told : they are what they 
fancy they are not, all the more for deceiving themselves ; 
for they are ignorant of the very thing it most concerns 
them to know — the penalty of injustice. This is not, as they 
imagine, stripes and death, which do not always fall on the 
E. wrong-doer, but a penalty that cannot be escaped. 
Theod. What penalty is that ? 

Socr. There are two patterns, my friend, in the unchange- 
able nature of things, one of divine happiness, the other of 
godless misery — a truth to which their folly makes them 

177. utterly blind, unaware that in doing injustice they are grow- 
ing less like one of these patterns and more like the other. 
The penalty they pay is the life they lead, answering to the 
pattern they resemble. But if we tell them that, unless 
they rid themselves of their superior cunning, that other 
region which is free from all evil will not receive them after 
death, but here on earth they will dwell for all time in some 
form of life resembling their own and in the society of things 
as evil as themselves, all this will sound like foolishness to 
such strong and unscrupulous minds. 
Theod. So it will, Socrates. 

b. Socr. I have good reason to know it, my friend. But 
there is one thing about them : when you get them alone 
and make them explain their objections to philosophy, then, 
if they are men enough to face a long examination without 
running away, it is odd how they end by finding their own 
arguments unsatisfying ; somehow their flow of eloquence 
runs dry, and they become as speechless as an infant. 

All this, however, is a digression ; we must stop now, 

c. and dam the flood of topics that threatens to break in and 
drown our original argument. With your leave, let us go 
back to where we were before. 

Theod. For my part, I rather prefer listening to your 
digressions, Socrates ; they are easier to follow at my time 
of life. However, let us go back, if you like. 

The tone of this digression goes beyond the Gorgias and the 
Phaedo and is far removed from the humanity of Socrates, 
who certainly knew the way to the market-place, though he 
deliberately kept out of politics. There is a foretaste of Cynicism 
in the emphatic contempt of wealth and high birth. The main 
contrast is not between the life of contemplation and the active life, 
to which, in a reformed society, the philosopher king would acknow- 
ledge his duty to descend. Many saints, like Teresa, have led very 



active lives without abandoning the joys of contemplation. The 
life contrasted with the philosopher's is at first that of the rhetorician, 
and towards the end that of the man trained in rhetoric to be the 
ruler of society, the strong-minded man who will stick at nothing 
and thinks himself a ' realist ' because he has no conception of the 
reality of ideals — a familiar figure in the post-war world of Plato's 
manhood, as in our own. It is an easy conjecture that some part 
of this tirade was inspired by Plato's experiences at the court of 

The allusions to the allegory of the Cave, the passage about the 
true meaning of kingship, happiness, and justice, are intended to 
recall the whole argument of the Republic, with its doctrine of the 
divine, intelligible region of Forms, the true objects of knowledge. 
This is no mere digression ; it indicates — what cannot be directly 
stated — the final cleavage between Platonism and the extreme 
consequences of the Protagorean thesis. The Theaetetus here opens 
a window upon the world of true being ; but the vision must be 
closed. Our concern at present is only with the world of appear- 
ances and its claim to yield knowledge. 

177C-179C. Refutation of the Defence of Protagoras 

The argument is now resumed at the point where it was dropped 
(172A), when the genuinely Protagorean position had been isolated 
from extraneous elements. That position is now stated again, to 
mark that we have been straying beyond it. Socrates proceeds to 
refute the defence he put forward earlier on Protagoras' behalf. 

177c. Socr. Very well. I think the point we had reached was 
this. We were saying that the believers in a perpetually 
changing reality and in the doctrine that what seems to 
an individual at any time also is for him would, in most 
matters, strongly imsist upon their principle, and not least 
in the case of what is right they would maintain that any 
D. enactments a State may decide on certainly are right for 
that State so long as they remain in force ; but when it 
comes to what is good, we said that the boldest would not 
go to the length of contending that whatever a State may 
believe and declare to be advantageous for itself is in fact 
advantageous for so long as it is declared to be so — unless, 
he meant that the name * advantageous ' would continue 
to be so applied; but that would be turning our subject 
into a joke. 
Theod. Certainly. 


THEAETETUS 1 77c-l 79c 

177E. Socr. We will suppose, then, that he does not mean the 
name, but has in view the thing that bears it. 
Theod. We will. 1 

Socr. Whatever name the State may give it, advantage 
is surely the aim of its legislation, and all its laws, to the 
full extent of its belief and power, are laid down as being 
for its own best profit. Or has it any other object in view 
when it makes laws ? 

178. Theod. None. 

Socr. Then does it also hit the mark every time ? Or 
does every State often miss its aim completely ? 
Theod. I should say that mistakes are often made. 
Socr. We may have a still better chance of getting every- 
one to assent to that, if we start from a question covering 
the whole class of things which includes the advantageous. 
It is, I suggest, a thing that has to do with future time. 
When we legislate, we make our laws with the idea that they 
will be advantageous in time to come. We may call this 
class ' what is going to be \ 

b. Theod. Certainly. 

Socr. Here, then, is a question for Protagoras or anyone 
else who agrees with him : According to you and your 
friends, Protagoras, man is the measure of all things — of 
white and heavy and light and everything of that sort. 
He possesses in himself the test of these things, and believing 
them to be such as he experiences them, he believes what 
is true and real for him. Is that right ? 
Theod. Yes. 
Socr. Is it also true, Protagoras (we shall continue), that 

c. he possesses within himself the test of what is going to be in 
the future, and that whatever a man believes will be, actually 
comes to pass for him who believes it ? Take heat, for 
example. When some layman believes that he is going to 
catch a fever 2 and that this hotness is going to exist, and 
another, who is a physician, believes the contrary, are we 
to suppose that the future event will turn out in accordance 
with one of the two opinions, or in accordance with both 
opinions, so that to the physician the patient will not be hot 
or in a fever, while he will be both these things to himself ? 

1 It is not a question of the State giving the name ' advantageous ' to any 
class of actions it enjoins. Legislation must be understood to imply a judg- 
ment that the conduct prescribed will have good effects. 

2 TTvperov is subject of Xrfipeodai, cf. Phaedr. 251 a, 'Bpajs kox 0€pp.oTr)s 
arjdrjs Aa/LtjSavet. 



178c. Theod. That would be absurd. 

Socr. And on the question whether a wine is going to be 

d. sweet or dry, I imagine the vine-grower's judgment is authori- 
tative, not a flute-player's. 

Theod. Of course. 

Socr. Or again, on the question whether a piece of music 

is going to be in tune or not, a gymnastic trainer would not 

have a better opinion than a musician as to what the trainer 

himself will later judge to be in good tune. 

Theod. By no means. 

Socr. And when a feast is being prepared, the guest who is 

to be invited, supposing him not to be an expert in cookery, 

will have a less authoritative opinion than the confectioner 

upon the pleasure that will result. We will not dispute yet 

e. about what already is or has been pleasant to any indivi- 
dual ; but about what will in the future seem and be to 
anyone, is every man the best judge for himself, or would 
you, Protagoras, — at least in the matter of the arguments 
that any one of us would find convincing for a court of law 
— have a better opinion beforehand than any untrained 
person ? 

Theod. Certainly, Socrates, in that matter he did emphati- 
cally profess to be superior to everybody. 
Socr. Bless your soul, I should think he did. No one 
179. would have paid huge sums to talk with him, if he had not 
convinced the people who came to him that no one whatever, 
not even a prophet, could judge better than he what was 
going to be and appear in the future. 
Theod. Quite true. 

Socr. And legislation, too, and the question of advan- 
tageousness are matters concerned with the future ; and 
everyone would agree that a State, when it makes its laws, 
must often fail to hit upon its own greatest advantage ? 
Theod. Assuredly. 

Socr. Then we may quite reasonably put it to your master 
b. that he must admit that one man is wiser than another and 
that the wiser man is the measure, whereas an ignorant 
person like myself is not in any way bound to be a measure, 
as our defence of Protagoras tried to make me, whether I 
liked it or not. 

Theod. I think that is the weakest point in the theory, 
Socrates, though it is also assailable in that it makes other 
people's opinions valid when, as it turns out, they hold 
Protagoras' assertions to be quite untrue. 


THEAETETUS 179c-181b 

179c. Socr. There are many other ways, Theodorus, of assailing 
such a position and proving that not every opinion of every 
person is true. 

The Defence of Protagoras is thus refuted. The argument which 
' turns the tables ' is reaffirmed by Theodorus ; and it has been 
shown that not all judgments can be true. When the patient and 
the doctor disagree about what the patient's experiences will be 
at some future time, they are disagreeing about the same fact, which 
is not at the moment part of the private experience of either, so 
that he might claim to be the only possible judge. They cannot 
both be right. No more can two politicians who dispute whether 
some law or decree will have good effects for the State. Protagoras' 
own profession as an educator of good citizens rested entirely on 
his claim to be a better judge than his pupils of what they would, 
when educated, find to be good for them. 

179C-181B. The extreme Heracleitean position, contrasted with Par- 
menides' denial of all motion and change 

Plato has now shown why he will not accept the Protagorean 
position as extended by its author to judgments which go beyond 
the individual's immediate and private experience of his present 
sensations. But within this narrower field he has himself accepted 
the position, and built it into his own account of the nature of per- 
ception. We must now return to that account and consider the 
second element, drawn from the flux doctrine of Heracleitus. With 
what reservations and restrictions are we to adopt the principle 
that all things are perpetually in motion ? 

179c. Socr. {continues). But with regard to what the individual 
experiences at the moment — the source of his sensations and 
the judgments in accordance with them — it is harder to 
assail the truth of these. Perhaps it is wrong to say 
' harder ' ; maybe they are unassailable, and those who 
assert that they are transparently clear 1 and are instances 
of knowledge may be in the right, and Theaetetus was not 
beside the mark when he said that perception and know- 
d. ledge were the same thing. 

We must, then, look more closely into the matter, as our 
defence of Protagoras enjoined, and study this moving 

1 Cf. Phaedrus 250c, ' through the clearest of the senses, sight, we apprehend 
beauty in the perfect clearness of its radiance ' (81a rrjs (vapyeardr-qs alodtfoecos 
oriXpov ivapycarara). Plato will contend that perception of sensible quali- 
ties, though infallible in the sense above defined, does not reveal true reality 
and is therefore not knowledge. 



179D. reality, ringing its metal to hear if it sounds true or cracked. 
However that may be, there has been no inconsiderable 
battle over it and not a few combatants. 
Theod. Anything but inconsiderable ; in Ionia, indeed, it 
is actually growing in violence. The followers of Heracleitus 
lead the quire of this persuasion with the greatest vigour. 
Socr. All the more reason, my dear Theodorus, to look into 
it carefully and to follow their lead by tracing it to its 
e. source. 

Theod. By all means. For there is no discussing these 
principles of Heracleitus — or, as you say, of Homer or still 
more ancient sages — with the Ephesians themselves, who 
profess to be familiar with them ; you might as well talk 
to a maniac. Faithful to their own treatises they are 
literally in perpetual motion ; their capacity for staying 
still to attend to an argument or a question or for a quiet 

180. interchange of question and answer amounts to less than 
nothing, or rather even a minus quantity is too strong an 
expression for the absence of the least modicum of repose 
in these gentry. 1 When you put a question, they pluck 
from their quiver little oracular aphorisms to let fly at you ; 
and if you try to obtain some account of their meaning, 
you will be instantly transfixed by another, barbed with 
some newly forged metaphor. You will never get anywhere 
with any of them ; for that matter they cannot get anywhere 
with one another, but they take very good care to leave 

b. nothing settled either in discourse or in their own minds ; 
I suppose they think that would be something stationary 
— a thing they will fight against to the last and do their 
utmost to banish from the universe. 

Socr. Perhaps, Theodorus, you have seen these gentlemen 
in the fray and never met them in their peaceable moments ; 
indeed they are no friends of yours. I dare say they keep 
such matters to be explained at leisure to their pupils whom 
they want to make like themselves. 
Theod. Pupils indeed ! My good friend, there is no such 

c. thing as a master or pupil among them ; they spring up like 
mushrooms. Each one gets his inspiration wherever he can, 
and not one of them thinks that another understands any- 
thing. So, as I was going to say, you can never bring them 

1 Taking to ouS' ovSev (' not even nothing ' = a minus quantity) as the 
subject of u7TcpjSaAAei, ' is excessive (an exaggerated estimate) with respect to 
the absence of even a little quietness in them '. For npds, cf. Soph. 258 a, 5 ; 
Phaedo 75 a, 9. 


THEAETETUS 179c-181b 

i8oc. to book, either with or without their consent. We must 
take over the question ourselves and try to solve it like a 

Socr. That is a reasonable proposal. As to this problem, 
then, have we not here a tradition from the ancients who 

d. hid their meaning from the common herd in poetical figures, 
that Ocean and Tethys, the source of all things, are flowing 
streams and nothing is at rest ; and do not the moderns, 
in their superior wisdom, declare the same quite openly, 
in order that the very cobblers may hear and understand 
their wisdom and, abandoning their simple faith that some 
things stand still while others move, may reverence those 
who teach them that everything is in motion ? 

But I had almost forgotten, Theodorus, another school 

e. which teaches just the opposite, that reality ' is one, immov- 
able : " Being " is the name of the All V and much else that 
men like Melissus and Parmenides maintain in opposition to 
all those people, telling us that all things are a Unity which 
stays still within itself, having no room to move in. How are 
we to deal with all these combatants ? For, little by little, our 

1 Reading olov (for olov), aKlmjrov reXedci. rep iravrl ovop.* elvcu. There is 
no reason to doubt that this verse stood in the text of Parmenides used 
by Plato and Simplicius, who twice quotes it, without reference to the Theae- 
tetus, at Phys. 29, 15 and 143, 8. Both must have understood it as above 
translated. The sense is good and relevant. I cannot believe that Plato 
concocted the verse from the two halves of frag. 8, 38, eVet to yc Molp* 
dncBrjaev J ovXov aKiVTjrov r* epevar rep irdvr(a) ovop.* eorai \ 600a fiporol Karidt-vro, 
kt\, which belong to different sentences and have a quite different meaning. 

I suggest, however, that Parmenides' text itself was corrupt. rcXedeiv 
is not used by the Pre-Socratics in the sense ' to be \ I conjecture re OeXei, 
and supply as the only possible subject of OeXa logical Necessity (*Avdyicq 
or Alter) or Molpa). Cf. Heracl. 65 : tv to ao<j>6v p.ovvov Xeyeodai ovk edeXei koI 
edeXei Z-qvos ovopa. The verse can then be placed after frag. 19 at the end 
of the poem : 

outco Tot Kara ho£av e<f>v rdSc #ccu wv earn 
Kal ficreTrcir* airo rov&e rcXevrrjaovoi rpa<f>evra' 
rots B* ovofx dvdpaiTTOi KareOcvr* €7rtcrr)iiov €Kaara>. 
Krovraiv ovSevl irlons €vl' jjlovvov ydp 'AvdyK7}> 
olov aKivr\r6v re deXci ru> Travrl ovofi elvai. 

* Men have given many names to changing things ; but all these names are 
false ; for Necessity is willing that the All should only be called one and 
immovable.' This makes a good ending. If we now suppose that the text 
used by Plato and Simplicius had been corrupted and corrected into fxovvov 
yap avdyKTj \ olov, aKLV-qrov reX^Oei. r<p Travrl ovop.* clvai, we have the verse 
quoted, independently and correctly, by Plato and Simplicius, as Par- 
menides' last word on the unity and changelessness of Being (see Classical 
Review, 1935, A New Fragment of Parmenides). 



i8oe. advance has brought us, without our knowing it, between 
the two lines ; and, unless we can somehow fend them off and 

181. slip through, we shall suffer for it, as in that game they play- 
in the wrestling schools, where the players are caught by 
both sides and dragged both ways at once across the line. 
The best plan, I think, will be to begin by taking a look at 
the party whom we first approached, the men of Flux ; and 
if there seems to be anything in what they say, we will 
help them to pull us over to their side and try to elude the 
others ; but if we find more truth in the partisans of the 
immovable whole, we will desert to them from these revolu- 
b. tionaries who leave no landmark unremoved. If both sides 
turn out to be quite unreasonable, we shall merely look 
foolish if we suppose that nobodies like ourselves can make 
any contribution after rejecting such paragons of ancient 
wisdom. Do you think it worth while to go further in the 
teeth of such danger, Theodorus ? 

Theod. Certainly, Socrates ; I could not bear to stop before 
we have found out what each of the two parties means. 

Theodorus' vigorous outburst perhaps expresses Plato's impatience 
with the later followers of Heracleitus, who appear to have copied 
with exaggeration their master's use of cryptic aphorisms and reiter- 
ated his doctrine of flux without contributing anything more than 
emphasis. The Heracleitean position that is to be examined is 
the extreme position, comparable to the equally extreme denial of 
all motion and change by Parmenides. Plato's own task was to 
discover what elements of truth each party was trying to express. 
Parmenides will be reserved for the Sophist. The Theaetetus, being 
concerned with the sensible world, deals with Heracleitus, whose 
doctrine has its application in that world. 

181B-183C. Criticism of extreme Heracleiteanism 

Socrates opens his criticism of Heracleitus by drawing the distinc- 
tion between two kinds of change : local motion and change of 
quality. At Parmenides 138B these were declared to be the only 
two species of change. The word for change of quality (aXXoiovodai) 
occurs in Heracleitus himself : ' God is day and night, winter and 
summer, war and peace, surfeit and hunger ; he changes {aXXoiovrai) 
just as fire, when blended with spices, is named according to the 
savour of each ' (36 Byw., 67 Diels). Whether the later Heraclei- 
teans drew this distinction or not, they appear to have denied any 
kind of rest or fixity. 


THEAETETUS 181b-183c 

i8ib. Socr. Well, if you feel so strongly about it, we must look 
into the matter. I think our study of change should begin 

c. with the question : What after all do they mean when they 
say all things are in change ? What I mean is this : Do 
they recognise one kind of change or two ? I think there 
are two ; but I must not be alone in my opinion ; you must 
take your share in the risk, so that we may meet together 
whatever fate shall befall us. Tell me : do you call it change 
when something removes from place to place or revolves in 
the same place ? 

Theod. Yes. 

Socr. Let that be one kind, then. Now suppose a thing 

d. stays in the same place but grows old or turns black instead 
of white or hard instead of soft or alters in some other 
way, isn't it proper to call that a different kind of change ? 
Theod. Yes, it must be. 

Socr. So I should recognise these as two kinds of change 
— alteration and local movement. 
Theod. And you are right. 

Socr. Having made that distinction, then, let us now 
begin our talk with these people who say that everything 
is in change and ask them : Do you say everything is in 
E. both sorts of change— both moving in place and altering 
— or that part changes in both ways, part in only one of the 
two ? 

Theod. I really cannot tell ; but I think they would say 
' in both ways \ 

Socr. Yes, my friend ; otherwise they will find things at 
rest as well as things in change, and it will be no more 
correct to say that everything is changing than to say that 
everything is at rest. 
Theod. Quite true. 

Socr. So, since they are to be in change and unchangingness 
182. must be impossible anywhere, all things are always in every 
kind of change. 
Theod. That follows. 

The theory of the nature of sense-perception, stated earlier, is now 
included in the position we are examining. Judgment, as distinct 
from sense-perception, has already been disposed of in the criticism 
of Protagoras. Being fallible, judgment (as Theaetetus will remark 
later, 187B) cannot be simply identified with knowledge. So the dis- 
cussion has now been narrowed down to the question : Can sense- 
perception, whose infallibility has been admitted, give us know- 



ledge ? Plato stands by his analysis of sense-perception, which is 
now recalled. It is still attributed to those more refined thinkers 
who have been alleged to hold the doctrine of flux. That doctrine 
was originally stated without any reservation as applying to ' all 
things \ Plato has now to point out that, if the objects of percep- 
tion (to which it does, in his opinion, apply) are taken to be ' all 
things ', there can be no such thing as knowledge at all, since no 
statement we make about these perpetually changing things can 
remain true for two moments together. All discourse will be im- 
possible, since there will be no fixed and stable things for our words 
to refer to. 

182A. Socr. Now consider this point in their theory. The 
account they gave of the genesis of hotness or whiteness or 
whatever it may be, we stated — didn't we ? — in this sort of 
way : that any one of these things is something that moves 
in place, simultaneously with a perception, between agent 
and patient ; and that the patient becomes perceptive, not 
a perception, while the agent comes to have a quality, rather 
than to be a quality. Perhaps this word ' quality ' strikes 
you as queer and uncouth and you don't understand it as 
a general expression * ; so let me give particular instances. 
b. The agent does not become hotness or whiteness, but hot 
or white, and so on with all the rest. No doubt you remem- 
ber how we put this earlier : that nothing has any being 
as one thing just by itself, no more has the agent or patient, 
but, as a consequence of their intercourse with one another, 
in giving birth to the perceptions and the things perceived, 
the agents come to be of such and such a quality, and the 
patients come to be percipient. 
Theod. I remember, of course. 

The reference is to the statement (156E) that ' white ', ' hot ', 
' hard ', etc., have no being just by themselves, and that the agent 
(as such) and the patient (as such) do not exist until the external 
object and the sense-organ come within range of one another and the 
' quick movements ' begin to pass between them. Such being the 
process of perception, Socrates now takes objects and perceptions 
separately, beginning with objects. 

1 This is the first occurrence in Greek of the substantive ttoiottjs, though the 
corresponding adjective 77-010?, ' of what sort ', or ' nature ' or ' character ', 
was in common use. The word was coined as a general term for all characters 
like ' hotness ', ' whiteness ', ' heaviness ', etc., the termination -tijs corre- 
sponding to ' -ness ' in English. 


THEAETETUS 181b-183c 

182c. Socr. Very well, then, we will not inquire into other parts 
of their theory, whether they mean this or that, but keep 
to the point we have in view and ask them this : All things, 
by your account, are in a perpetual stream of change. Is 
that so ? 
Theod. Yes. 

Socr. With both the kinds of change we distinguished — 
both moving in place and altering ? 

Theod. Certainly, if they are to be completely in change. 
Socr. Well now, if they only moved in place without alter- 
ing in quality, we should be able to say what qualities they 
have as they move in this stream, shouldn't we ? 
Theod. Yes. 

D. Socr. Since, however, there is nothing constant here either 
— the flowing thing does not flow white but changes, so that 
the very whiteness itself flows and shifts into another colour, 
in order that the thing may escape the charge of constancy 
in that respect — can we ever give it the name of any colour 
and be sure that we are naming it rightly ? 

Theod. How can that be done, Socrates ? Or how can 
anything else of the kind you mean be called by its right 
name, if, while we are speaking, it is all the time slipping 
away from us in this stream ? 

Socr. And again, what are we to say of a perception of 
any sort ; for instance, the perception of seeing or hearing ? 

E. Are we to say that it ever abides in its own nature as seeing 
or hearing ? 

Theod. It certainly ought not, if all things are in change. 
Socr. Then it has no right to be called seeing, any more 
than not-seeing, nor is any other perception entitled to be 
called perception rather than not-perception, if everything 
is changing in every kind of way. 
Theod. No, it hasn't. 

Socr. And moreover perception is knowledge, according 
to Theaetetus and me. 
Theod. Yes, you did say so. 

Socr. In that case, our answer to the question, what 
knowledge is, did not mean knowledge any more than not- 
183. Theod. So it appears. 

The latter part of this argument, dealing with perception, seems 
at first sight less cogent than the part concerned with objects. It 
might be objected that, though the organ of sight and the percep- 



tion (seeing) may be changing all the time, that does not mean that 
seeing ceases to be seeing and might as well be called ' not-seeing \ 
Theaetetus' identification of perception with knowledge meant that 
every individual act of perception is infallible awareness of some- 
thing that exists. This is not disproved by pointing out that the 
perception and its object are always changing. The total complex 
— perception + object — may be changing, but if it yields know- 
ledge at any moment, it does so at all moments. We are merely 
aware of slightly different objects in a slightly different way from 
moment to moment ; but each new perception is just as infallible 
as the last. The fact of change does not make perception cease to 
be perception, or, if it ever is knowledge, cease to be knowledge. 

The extreme Heracleitean, however, cannot make this reply. 
It would mean that my perception, though changing in content, 
remains the same in so far as it always has the character of being 
perception and knowledge. But the Heracleitean says that nothing 
ever remains the same. Plato's point is that, if ' all things ' without 
exception are always changing, language can have no fixed meaning. 
In the statement ' Perception is knowledge ' the meanings of the 
words must be constantly shifting. So the statement cannot remain 
true or the same statement. 

The Heracleitean Cratylus, who influenced Plato in his youth, 
did in fact reach this conclusion. Aristotle says that thinkers who 
identified the real with the sensible world concluded that ' to seek 
truth would be to chase a flying bird \ ' They saw that all this 
world of nature is in movement and that about that which changes 
no true statement can be made ; at least, regarding that which 
everywhere in every respect is changing nothing could be truly 
affirmed. It was this belief that blossomed into the most extreme 
of the views above mentioned, that of the professed Heracleiteans, 
such as was held by Cratylus, who finally did not think it right to 
say anything but only moved his finger, and criticised Heracleitus 
for saying that it is impossible to step twice into the same river ; 
for he thought one could not do it even once.' x The conclusion 
Plato means us to draw is this : unless we recognise some class of 
knowable entities exempt from the Heracleitean flux and so capable 
of standing as the fixed meanings of words, no definition of know- 
ledge can be any more true than its contradictory. Plato is deter- 
mined to make us feel the need of his Forms without mentioning 
them. Without the Forms, as his Parmenides said, 2 there can be no 
discourse. The same conclusion had already been stated at the 
end of the Cratylus. 

1 At., Metaph. ioioa, 7, trans. Ross. 

2 See Introd., p. 11. 


THEAETETUS 181b-183c 

183A. Socr. That would be a pretty result of the improvement 
we made upon that first answer, 1 when we were so eager 
to prove it right by showing that everything is in change. 
Now it seems that what has in fact come to light is that, 
if all things are in change, any answer that can be given to 
any question is equally right : you may say it is so and it 
is not so — or ' becomes ', if you prefer to avoid any term 
that would bring these people to a standstill. 
Theod. You are right. 

Socr. Except, Theodorus, that I used the words ' so ' and 
' not so ', whereas we have no right to use this word ' so ' — 
what is ' so ' would cease to be in change — nor yet ' not so ' : 

B. there is no change in that either. Some new dialect will 
have to be instituted for the exponents of this theory, since, 
as it is, they have no phrases to fit their fundamental prop- 
osition — unless indeed it were ' not even no-how \ 2 That 
might be an expression indefinite enough to suit them. 
Theod. A most appropriate idiom. 

Socr. So, Theodorus, we are quit of your old friend, and 
not yet ready to concede to him that every man is the 

c. measure of all things, if he is not a wise man. Also, we 
shall not admit that knowledge is perception, at least on the 
basis of the theory that all things are in change, unless 
Theaetetus has some objection. 

Theod. That is excellent, Socrates ; for now these ques- 
tions are disposed of, it was agreed that I should be quit of 
answering your questions, as soon as the discussion of 
Protagoras' theory should come to an end. 

Two conclusions are here carefully stated. By the argument 
that the wise man is a better judge of what will be in the future 
we have disposed of Protagoras' doctrine as extended to judgments ; 
but in the restricted sphere of sense-perception our application of his 
principle still stands. Theaetetus* proposition, that perception is 
knowledge, has been refuted ' on the basis of the theory that all things 

1 Viz. that knowledge is the same as perception. 

2 The text is corrupt. oi58' ovtcds (W) cannot be right, since ovx ovrco has 
already been rejected as not indefinite enough. ou8' ows (BT) is not Greek for 
' No-how ' (oi38' ottojoovv, or ottokjtiovv) . If some still more negative expression 
is needed — ' not even nohow ' (cf. to ov& ovSeV, 180 a) — we might conjecture 
ov8' ovbeTTcos, a form as possible as ovScttotc or ovhcira>, which Plato might coin 
for this occasion (ov-ncos being poetic). Another possibility is ov<Kot>& 
onws, nescio quomodo, involving a pun on direipov = ' indefinite ' and drreipov = 
' ignorant' (as at Tim. 55c and Phileb. 17E). Pending a better suggestion, 
8* ovtws after fidXiora should be retained. 



are in change ' — the extreme Heracleitean position — but only on 
that basis. The theory of the nature of perception is not abandoned ; 
on the contrary it is used to disprove the claim of perception to be 
knowledge. It is true that the organs and objects of perception 
are always changing ; and if this were (as Theaetetus held) the only 
form of cognition, there would be no knowledge. Knowledge 
requires terms that will have a fixed meaning and truths that will 
remain true. 

The upshot of this section is that Plato has disentangled the 
application of the flux doctrine to sensible things, which he accepts, 
from the unrestricted assertion, ' All things whatsoever are in 
change ', which he rejects. The conclusion would be more obvious, 
if it were not his plan to exclude mention of the Forms — the things 
which are not in change and can be known. 

1830184B. Interlude. Socrates declines to criticise Parmenides 

Socrates now declines to discuss the equally extreme Eleatic 
doctrine that all motion and change is an illusion. The criticism 
of Parmenides is reserved for the Sophist, where the world of un- 
changing reality will be allowed to come into view. 

183c. Theaet. No, Theodorus, you must not be released until 

D. you and Socrates, as you proposed just now, have discussed 
those others who assert that the whole of things is at rest. 
Theod. Would you teach your elders, Theaetetus, to dis- 
honour their agreements ? No, for what remains you must 
prepare yourself to carry on the argument with Socrates. 
Theaet. Yes, if he wishes ; though I would much rather 
have been a listener while this subject is discussed. 
Theod. To invite Socrates to an argument is like inviting 
cavalry to fight on level ground. You will have something 
to listen to, if you question him. 

Socr. Well, but, Theodorus, I think I shall not comply 

E. with Theaetetus* request. 

Theod. Not comply ? What do you mean ? 
Socr. A feeling of respect keeps me from treating in an 
unworthy spirit Melissus and the others who say the uni- 
verse is one and at rest ; but there is one being x whom I 
respect above all : Parmenides himself is in my eyes, as 
Homer says, a ' reverend and awful ' figure. I met him when 
I was quite young and he quite elderly, and I thought there 

1 I suspect a sort of pun on eva ovra Ilaptievihrjv and the cv 6v he believed 
in. (So Dies, p. 123.) 


THEAETETUS 184b-186e 

184. was a sort of depth in him that was altogether noble. 1 I 
am afraid we might not understand his words and still less 
follow the thought they express. Above all, the original 
purpose of our discussion — the nature of knowledge — 
might be thrust out of sight, if we attend to these impor- 
tunate topics that keep breaking in upon us. In par- 
ticular, this subject we are raising now is of vast extent. 
It cannot be fairly treated as a side issue ; and an adequate 
handling would take so long that we should lose sight of 
our question about knowledge. Either course would be 
wrong. My business is rather to try, by means of my 
b. midwife's art, to deliver Theaetetus of his conceptions 
about knowledge. 
Theod. Well, do so, if you think that best. 

184B-186E. ' Perception is Knowledge ' finally disproved 

Plato has now eliminated those elements in Protagoras' doctrine 
and in Heracleiteanism which he will not accept. There remain 
those which he does accept and has included in his own theory 
of the nature of perception. He can now consider the claim of 
perception to be identical with knowledge. This claim, as advanced 
by Theaetetus, strictly implies not only that perception is know- 
ledge, but that it is the whole of knowledge. The following refuta- 
tion proves (1) that perception cannot be the whole of knowledge, 
for a great part of what is always called knowledge consists of 
truths involving terms which are not objects of perception ; and 
(2) that, even within its own sphere, the objects of perception 
have not that true reality which the objects of knowledge must 
possess. Hence, so far from being co-extensive with knowledge, 
perception is not knowledge at all. 

(1) Perception is not the whole of knowledge. — The first argument 
does not depend on the details of Plato's theory of sense-perception. 
Such a theory, he would hold, can never be more than a probable 
account which might need amendment. But even if it be not 
accepted, he can still show that perception, in the strict sense 
which is taken to exclude judgment, cannot be the whole of 

184B. Socr. Well then, Theaetetus, here is a point for you to 
consider. The answer you gave was that knowledge is 
perception, wasn't it ? 
Theaet. Yes. 

1 For this reference to the meeting described in the Parmenides, see Introd.,. 
p. 1. 



184B. Socr. Now suppose you were asked : ' When a man sees 
white or black things or hears high or low tones, what 
does he see or hear with ? ' I suppose you would say : 
' With eyes and ears \ 
Theaet. Yes, I should. 

c. Socr. To use words and phrases in an easy-going way 
without scrutinising them too curiously is not, in general, 
a mark of ill-breeding ; on the contrary there is something 
low-bred in being too precise. But sometimes there is no 
help for it, and this is a case in which I must take exception 
to the form of your answer. Consider : is it more correct 
to say that we see and hear with our eyes and ears or through 
them ? 

Theaet. I should say we always perceive through them, 
rather than with them. 

d. Socr. Yes ; it would surely be strange that there should 
be a number of senses ensconced inside us, like the warriors 
in the Trojan horse, and all these things should not con- 
verge and meet in some single nature — a mind, or what- 
ever it is to be called — with which we perceive all the objects 
of perception through the senses as instruments. 
Theaet. Yes, I think that is a better description. 
Socr. My object in being so precise is to know whether 
there is some part of ourselves, the same in all cases, with 
which we apprehend black or white through the eyes, and 

E. objects of other kinds through the other senses. Can you, 
if the question is put to you, refer all such acts of appre- 
hension to the body ? Perhaps, however, it would be 
better you should speak for yourself in reply to questions, 
instead of my taking the words out of your mouth. Tell 
me : all these instruments through which you perceive 
what is warm or hard or light or sweet are parts of the 
body, aren't they ? — not of anything else. 
Theaet. Of nothing else. 

Socr. Now will you also agree that the objects you per- 
185. ceive through one faculty cannot be perceived through 
another — objects of hearing, for instance, through sight, 
or objects of sight through hearing ? 
Theaet. Of course I will. 

Socr. Then, if you have some thought about both objects 
at once, you cannot be having a perception including both 
at once through either the one or the other organ. 
Theaet. No. 

Socr. Now take sound and colour. Have you not, to 


THEAETETUS 184b-186e 

185. begin with, this thought which includes both at once — that 
they both exist ? 
Theaet. I have. 

Socr. And, further, that each of the two is different from 
the other and the same as itself ? 

b. Theaet. Naturally. 

Socr. And again, that both together are two, and each 

of them is one ? 

Theaet. Yes. 

Socr. And also you can ask yourself whether they are 

unlike each other or alike ? 

Theaet. No doubt. 

Socr. Then through what organ do you think all this 

about them both ? What is common to them both cannot 

be apprehended either through hearing or through sight. 

Besides, here is further evidence for my point. Suppose 

it were possible to inquire whether sound and colour were 

both brackish or not, no doubt you could tell me what 

c. faculty you would use — obviously not sight or hearing, 
but some other. 

Theaet. Of course : the faculty that works through the 

Socr. Very good. But now, through what organ does 
that faculty work, which tells you what is common not 
only to these objects but to all things — what you mean 
by the words * exists ' and ' does not exist ' and the other 
terms applied to them in the questions I put a moment 
ago ? What sort of organs can you mention, corresponding 
to all these terms, through which the perceiving part of us 
perceives each one of them ? 

Theaet. You mean existence and non-existence, likeness 
and unlikeness, sameness and difference, and also unity 

D. and numbers in general as applied to them ; and clearly 
your question covers ' even ' and ' odd ' and all that kind 
of notions. You are asking, through what part of the body 
our mind perceives these ? 

Socr. You follow me most admirably, Theaetetus ; that 
is exactly my question. 

Theaet. Really, Socrates, I could not say, except that 
I think there is no special organ at all for these things, as 
there is for the others. It is clear to me that the mind 

E. in itself is its own instrument for contemplating the common 
terms that apply to everything. 
Socr. In fact, Theaetetus, you are handsome, not ugly 




185E. as Theodorus said you were ; for in a discussion handsome 
is that handsome does. And you have treated me more 
than handsomely in saving me the trouble of a very long 
argument, if it is clear to you that the mind contemplates 
some things through its own instrumentality, others through 
the bodily faculties. That was indeed what I thought 
myself ; but I wanted you to agree. 

186. Theaet. Well, it is clear to me. 

In this argument, for the first time, we go behind the earlier 
account of sense-perception, which regarded the subject as no 
more than a bundle of distinct sense-organs, and sense-perception 
as a process occurring between organ and external object. That 
account stands ; but it is now added that, behind the separate 
organs, there must be a mind, centrally receiving their several 
reports and capable of reflecting upon the data of sense and making 
judgments. In these judgments the thinking mind uses terms 
like ' exists ', ' is the same as ', ' is different from ', which are not 
objects of perception reaching the mind through the channel of 
any special sense, but are ' common ' to all the objects of sense. 
The mind gains its acquaintance with the meaning of such terms 
through its own instrumentality, not by the commerce between 
bodily organs and objects. 

These terms are called ' common ' (xoivd) in contrast with the 
' private ' (Idia) or ' peculiar ' objects of the several senses. 
' Common ' means no more than that. They are not to be con- 
fused with the ' common sensibles ' which Aristotle regarded as 
the objects of a common sensorium seated in the heart, namely 
objects perceptible by more than one sense, such as motion, shape, 
number, size, time. Plato does not speak of a ' common sense ' 
(xoivrj aioOrjoig), but on the contrary insists that his common terms 
are apprehended, not by any sense, but by thought. The judg- 
ments involving them are made by the mind, thinking by itself, 
without any special bodily organ. The terms are ' common ', 
not in Aristotle's sense, but in the sense in which a name is common 
to any number of individual things. Thus ' exists ' is ' applied 
in common to all things ' (xoivov em naoi, 185c) ; it can occur 
in a statement about any subject you like. Existence, we are 
presently told (i86a), ' attends on ' or ' belongs to 1 ' all things. 
These common terms are, in fact, the meanings of common names 
— what Plato calls ' Forms ' or ' Ideas \ The instances given here 
correspond to the instances given by Socrates in the Parmenides 
(129D), where he says that Zeno's dilemmas could be escaped by 
1 separating apart by themselves Forms such as likeness and un- 


THEAETETUS 184b-186e 

likeness, plurality and unity, rest and motion and all such things \ 
The terms there mentioned happen to be those which occurred in 
Zeno's arguments against plurality and motion ; Socrates adds 
later (130B) the moral Forms ' beautiful, good, and all such things ', 
just as he will presently add them here (186A). 1 In the Theaetetus 
Plato is determined to say as little as possible about the Forms, 
and he here avoids using the word ; but that these ' common ' 
terms simply are Forms should be obvious to anyone who has 
read the Parmenides. The avoidance of the word has misled many 
critics into asserting that the Forms are not mentioned in the 
Theaetetus, and miscalling these common terms ' categories \ 2 

Plato could not press the argument further in this direction 
without openly discussing the Forms as the true objects of know- 
ledge. But the inference is clear : that percepts cannot be the 
only objects of knowledge, as the identification of knowledge with 
perception implied. Any statement we can make about the objects 
of perception, and therefore any truth, must contain at least one 
of these common terms. Therefore all knowledge of truths, as 
distinct from immediate acquaintance with sense-data, involves 
acquaintance with Forms, which are not private objects of per- 
ception, not individual existents, not involved in the Heracleitean 
flux. The reader can now draw the first conclusion : Perception 
is not the whole of knowledge. 

The argument next proceeds to the second conclusion : (2) Per- 
ception, even within its own sphere, is not knowledge at all. 

i86a. Socr. Under which head, then, do you place existence ? 
For that is, above all, a thing that belongs to everything. 
Theaet. I should put it among the things that the mind 
apprehends by itself. 

Socr. And also likeness and unlikeness and sameness and 
difference ? 
Theaet. Yes. 

Socr. And how about ' honourable ' and ' dishonourable ' 
and ' good ' and ' bad ' ? 
Theaet. Those again seem to me, above all, to be things 

1 See Introd., p. 8. 

2 The entirely gratuitous confusion, traceable to Plotinus, of Plato's common 
terms with Aristotle's categories will be dealt with later (p. 274), where some 
of the common terms come up again for discussion. The moderns add a 
further confusion with the quite different use of ' category ' by Kant and 
others. Campbell (p. liii), for instance, speaks of ' necessary forms of thought 
which are as inseparable from perception as from reasoning '. The common 
terms are not forms of thought, but objects of thought (votjtcl), and they are 
separable from perception. 



I 86a. whose being is considered, one in comparison with another, 
by the mind, when it reflects within itself upon the past 

b. and the present with an eye to the future. 1 

Socr. Wait a moment. The hardness of something hard 

and the softness of something soft will be perceived by 

the mind through touch, will they not ? 

Theaet. Yes. 

Socr. But their existence and the fact that they both 

exist, and their contrariety to one another and again the 

existence of this contrariety are things which the mind 

itself undertakes to judge for us, when it reflects upon them 

and compares one with another. 

Theaet. Certainly. 

Socr. Is it not true, then, that whereas all the impressions 

which penetrate to the mind through the body are things 

c. which men and animals alike are naturally constituted to 
perceive from the moment of birth, reflections about them 
with respect to their existence and usefulness only come, 
if they come at all, with difficulty through a long and 
troublesome process of education ? 

Theaet. Assuredly. 

Socr. Is it possible, then, to reach truth when one cannot 

reach existence ? 

Theaet. It is impossible. 

Socr. But if a man cannot reach the truth of a thing, 

can he possibly know that thing ? 

d. Theaet. No, Socrates, how could he ? 

Socr. If that is so, knowledge does not reside in the 

impressions, but in our reflection upon them. It is there, 

seemingly, and not in the impressions, that it is possible 

to grasp existence and truth. 

Theaet. Evidently. 

Socr. Then are you going to give the same name to two 

things which differ so widely ? 

Theaet. Surely that would not be right. 

Socr. Well then, what name do you give to the first one 

— to seeing, hearing, smelling, feeling cold and feeling warm ? 

1 Theaetetus seems to be thinking of the recent argument against Protag- 
oras, turning on the question of judgments about the comparative goodness 
or badness of future effects, and what will seem honourable (laudable) or 
dishonourable customs to a State. Socrates stops him short and applies his 
statement to the contrasts of sense qualities. Touch can show us that this 
is hard, that soft ; but it is thought, not sense, that reflects upon the contrast 
of hard and soft. 


THEAETETUS 184b-186e 

i86e. Theaet. Perceiving. What other name is there for it ? 
Socr. Taking it all together, then, you call this perception ? 
Theaet. Necessarily. 

Socr. A thing which, we agree, has no part in apprehending 
truth, since it has none in apprehending existence. 
Theaet. No, it has none. 

Socr. Nor, consequently, in knowledge either. . 
Theaet. No. 

Socr. Then, Theaetetus, perception and knowledge cannot 
possibly be the same thing. 

Theaet. Evidently not, Socrates. Indeed, it is now perfectly 
plain that knowledge is something different from perception. 

Such is the final disproof of the claim of perception to be know- 
ledge. Though admitted to be, in a sense, infallible, perception 
has not the second mark of knowledge : it cannot apprehend 
existence and truth. There is a certain ambiguity about the words 
' existence ' (ovala) and ' truth ' (ahfjdeia) : both are commonly 
used by Plato to mean that true reality which he ascribes to Forms 
and denies to sensible objects. If we keep to the sense suggested 
by the previous context, the statement should mean that the 
simplest judgment, such as ' Green exists here \ is beyond the 
scope of perception proper, our immediate awareness of green. 
The faculty of perception has no cognizance of the meaning of the 
word ' exists ' ; and, since only judgments or statements can be 
true, all truths are beyond its scope. 

To the Platonist, however, who is familiar with the associations 
of ' reality ' and ' truth ', the passage will mean more than this. 
The statement that reflections on the existence or usefulness of 
our sense-impressions come only, if at all, after a long and trouble- 
some education seems at first sight to conflict with the argument 
for Recollection in the Phaedo, where it was asserted that from the 
time when we first begin to use our senses we make judgments 
involving Forms, which we must therefore have known before 
birth. All judgments involve the use of some common term ; and 
Plato cannot mean to deny here that uneducated people make 
judgments. Plainly he means that they have not such knowledge 
of Forms as the dialectician gains by the long process of education 
described in Republic vii. And the Phaedo may only mean that, 
though children do make judgments such as ■ This is like that ' 
and mean something by them, they have only a dim and confused 
apprehension of Forms such as likeness. The advance to knowledge 
is a gradual recovery of clear vision, possible only by a training in 



The conclusion suggested earlier was that perception cannot be 
the whole of knowledge because there are other objects — the com- 
mon terms — which the mind must know if it is to reflect at all. 
If we now take account of the Platonic sense of ' reality and truth ', 
we can add a further inference. Even my direct perception of my 
own sense-object cannot be called ' knowledge \ because the object 
is not a thing which is unchangingly real, but only something that 
becomes and is always changing. Some might say that they are 
more certain of the sensations and perceptions they have at any 
moment than they are of anything else ; and to deny the name 
of knowledge to such direct acquaintance is, in a sense, a matter 
of terminology. But to Plato knowledge, by definition, has the 
real for its object, and these objects have not true and permanent 
being. This point, however, cannot be elaborated without entering 
on an account of the intelligible world. Hence a certain ambiguity 
is allowed to remain about the meaning of ' reaching truth (reality) 
and existence '. 

II. The Claim of True Judgment to be Knowledge 

187A-C. Theaetetus states the claim of True Judgment 

In the foregoing argument against Protagoras the distinction 
between direct perception and judgment has gradually emerged. 
Theaetetus has been led to see that knowledge must be sought above 
the level of mere sensation or perception, somewhere in the field 
of that ' thinking ' or ' judging ' which has been described as an 
activity of the mind ' by itself ', exercised upon the reports of the 
senses and using the common terms. Judgments may be true or 
false. Theaetetus* next suggestion is that any judgment that is 
true is entitled to be called knowledge. 

187A. Socr. But when we began our talk it was certainly not 
our object to find out what knowledge is not, but what 
it is. Still, we have advanced so far as to see that we must 
not look for it in sense-perception at all, but in what goes 
on when the mind is occupied with things by itself, whatever 
name you give to that. 

Theaet. Well, Socrates, the name for that, I imagine, is 
' making judgments \ 

Socr. You are right, my friend. Now begin all over 
b. again. Blot out all we have been saying, and see if you 
can get a clearer view from the position you have now 
reached. Tell us once more what knowledge is. 
Theaet. I cannot say it is judgment as a whole, because 



187B. there is false judgment ; but perhaps true judgment is 
knowledge. You may take that as my answer. If, as we 
go further, it turns out to be less convincing than it seems 
now, I will try to find another. 

Socr. Good, Theaetetus ; this promptness is much better 
than hanging back as you did at first. If we go on like 
c. this, either we shall find what we are after, or we shall 
be less inclined to imagine we know something of which 
we know nothing whatever ; and that surely is a reward 
not to be despised. And now, what is this you say : that 
there are two sorts of judgment, one true, the other false, 
and you define knowledge as judgment that is true ? 
Theaet. Yes ; that is the view I have come to now. 

The word (dot~a£eiv) above translated ' making judgments ' has 
been loosely used earlier for thinking or reflection of any sort 
that goes on in the mind ' by itself \ Judgment (dotja) will be more 
precisely defined presently (190A) as the decision terminating the 
mind's inward debate with itself. But the verb continues to be 
used as a synonym for thinking generally and even for ' thinking 
of ' some object. The translation will. follow Plato in using what- 
ever expression seems most natural in each context. 

187C-E. How is false judgment possible ? 

Instead of developing and criticising Theaetetus' new suggestion, 
Socrates here goes back to a point that arose in the Defence of 
Protagoras. Almost the whole of this section of the dialogue will 
be devoted to attempts to account for the possibility of false 
judgment. At 167A Protagoras said that no one can judge falsely ; 
' for it is not possible either to think the thing that is not or to 
think anything but what one experiences, and all experiences are 
true '. So far, our only reply to this has been to argue ad hominem 
that if all judgments are true, Protagoras refutes himself, and 
that two contradictory judgments about a future fact which is not 
now part of ' what one experiences ', cannot both be true. We 
have not shown that it is possible to ' think the thing that is not ' ; 
and if it is not possible, Protagoras could reply that then all judg- 
ments must be true and his position is unassailable by such 

In the next dialogue, the Sophist whom we attempt to define 
will be found taking refuge in this position ; and he is not finally 
dislodged from it till near the end, where the introduction of the 
theory of Forms at last provides a satisfactory definition of false 
statement and judgment. The Theaetetus is leaving the Forms out 



of account so far as possible, and the long analysis here given of the 
problem of false judgment cannot, accordingly, yield a complete 
solution. Its object is to explore the ground within the field of the 
present discussion and to see how far we can get towards an explan- 
ation of false judgment without invoking the Forms. 

187c. Socr. Then, had we better go back to a point that came 
up about judgment ? 
Theaet. What point do you mean ? 

D. Socr. A question that worries me now, as often before, 
and has much perplexed me in my own mind and also in 
talking to others. I cannot explain the nature of this 
experience we have, or how it can arise in our minds. 
Theaet. What experience ? 

Socr. Making a false judgment. At this moment I am 
still in doubt and wondering whether to let that question 
alone or to follow it further, not as we did a while ago, but 
in a new way. 
, Theaet. Why not, Socrates, if it seems to be in the least 
necessary ? Only just now, when you and Theodorus were 
speaking of leisure, you said very rightly that there is no 
pressing hurry in a discussion of this sort. 

E. Socr. A good reminder ; for this may be the right moment 
to go back upon our track. It is better to carry through 
a small task well than make a bad job of a big one. 
Theaet. Certainly it is. 

187E-188C. False Judgment as thinking that one thing {known or 
unknown) is another thing [known or unknown) 

Socrates opens up this new problem with two arguments showing 
that false judgment cannot be explained if we limit the discussion 
to the terms in which it was commonly debated by contemporary 
Sophists. Plato, as often, begins with a simple and naive view 
which ignores certain relevant factors, and gradually brings these 
factors in. The whole discussion, however, as we shall see, is limited 
by certain fundamental premisses, which are not Plato's own. He 
is criticising other people's attempts to account for the existence of 
false judgments, and the conclusion is negative : they have failed 
to explain it, and must fail so long as those premisses are assumed. 

(1) If we accept the dilemma that anything must be either 
known to us or (totally) unknown, it is hard, Socrates argues, to 
see how we can ever think that one thing (whether known to us or 
not) can be another thing (whether known to us or not), i.e. mistake 
one thing for another. 


THEAETETUS 187e-188c 

187E. Socr. How shall we set about it, then ? What is it that 
we do mean ? Do we assert that there is in every case a 
false judgment, and that one of us thinks what is false, 
another what is true, such being the nature of things ? 
Theaet. Certainly we do. 
188. Socr. And, in each and all cases, it is possible for us either 
to know a thing or not to know it ? I leave out of account 
for the moment becoming acquainted with things and for- 
getting, considered as falling between the two. Our argu- 
ment is not concerned with them just now. 
Theaet. Well then, Socrates, there is no third alternative 
left in any case, besides knowing and not knowing. 
Socr. And it follows at once that when one is thinking 
he must be thinking either of something he knows or of 
something he does not know ? 
Theaet. Necessarily. 

Socr. And further, if you know a thing, you cannot also 
B. not know it ; and if you do not know it, you cannot also 
know it ? 

Theaet. Of course. 1 

Socr. Then is the man who thinks what is false supposing 
that things he knows are not those things but other things 
he knows, so that, while he knows both, he fails to recognise 
either ? 2 

Theaet. No, that is impossible, Socrates. 
Socr. Well then, is he supposing that things he does not 
know are other things he does not know ? Is this possible — 
that a man who knows neither Theaetetus nor Socrates 
should take it into his head that Socrates is Theaetetus or 
Theaetetus Socrates ? 
c. Theaet. No. How could he ? 

Socr. But surely a man does not imagine that things he 

does know are things he does not know, or that things he 

does not know are things he knows ? 

Theaet. No, that would be a miracle. 

Socr. What other way is there, then, of judging falsely ? 

There is, presumably, no possibility of judging outside these 

alternatives, granted that everything is either known by us 

1 This apparently obvious admission is retracted later (191 a). There is 
a sense in which you do not know (are not now conscious of) what you do 
know (have become acquainted with and possess stored somewhere in your 
memory) . 

* ayvoelv means both ' fail to recognise ' and ' be ignorant of. No English 
expression covers both meanings. 



1 88c. or not known ; and inside them there seems to be no room 
for a false judgment. 
Theaet. Quite true. 

The limitations of this argument are obvious. As the illustration 
shows, ' to know ' is used in the sense in which I am said to know, 
not a truth, but a person or an object formerly seen and now remem- 
bered. We can divide all things into those we know in this sense 
and those we do not ; and we can ignore any processes of becoming 
acquainted and forgetting. The argument is that I cannot think 
that a friend is a total stranger, or that one stranger is another 
stranger, or that one friend is another friend. False judgments are 
never of that pattern. Three points are to be noted. 

(i) The field is limited to judgments of the form asserting that 
one thing is (identical with) another — that Theaetetus is Socrates. 
Very few false judgments consist in mistaking one thing for another ; 
but this limitation was characteristic of sophistic discussion of the 
question, partly because, as Apelt observes, the formula ' one thing 
is another ' (to irsQov steqov elvai) was the Greek equivalent for 
our ' % is A ', where x is subject, A predicate. This led to the 
confusion of commoner types of proposition with assertions of 
identity. It is not to be supposed, however, that Plato was guilty 
of this confusion. 

(2) The discussion is psychological, rather than logical. It is 
argued that we never in fact think that Theaetetus whom we know 
is Socrates whom we also know. It is true that when two known 
objects are clearly before the mind we do not judge that one is the 
other. Logicians, however, might maintain that there is a false 
' proposition ' : ' Theaetetus is identical with Socrates \ which 
has a meaning, though I cannot believe it. With that we are not 
concerned, but only with judgments and statements that can be 
actually made and believed by some rational being. Plato never 
discusses ' propositions ' that no one propounds. 1 

(3) When we come to objects that are unknown (things I have 
never been acquainted with), it may be urged that I can identify 
one unknown object with another : I can judge (truly or falsely) 
that Sir Philip Francis was the author of the Letters of Junius. 
Nearly all historical knowledge is about things unknown to us in 
the present sense. But the argument assumes that, unless I ' know ' 
an object, my mind must be a complete blank with respect to it, 
as it is with respect to a person I have never seen or heard of. 

1 Hence in translating Plato the unhappy word ' proposition ' should be 
avoided where modern associations are likely to obtrude themselves. See 
below, p. 265. 


THEAETETUS 188c-189b 

Plato was not blind to these considerations. The only conclusion, 
so far, is that so long as we confine the question to these very narrow 
limits, we cannot explain the occurrence of false judgment. 

188C-189B. False Judgment as thinking the thing that is not 

The second argument develops the current objection to the 
possibility of ' thinking the thing that is not ' — a phrase which 
Protagoras used as equivalent to ' judging falsely ' (167 a). 

188c. Socr. Perhaps, then, we had better approach what we 
are looking for by way of another alternative. Instead of 

D. ' knowing or not knowing ', let us take ' being or not 
being \ 

Theaet. How do you mean ? 

Socr. May it not simply be that one who thinks what is not 
about anything cannot but be thinking what is false, what- 
ever his state of mind may be in other respects ? 
Theaet. There is some likelihood in that, Socrates. 
Socr. Then what shall we say, Theaetetus, if we are asked : 
' But is what you describe possible for anyone ? Can any 
man think what is not, either about something that is or 
absolutely ? ' I suppose we must answer to that : ' Yes, 

E. when he believes something and what he believes is not 
true/ Or what are we to say ? 

Theaet. We must say that. 

Socr. Then is the same sort of thing possible in any other 
case ? 

Theaet. What sort of thing ? 

Socr. That a man should see something, and yet what he 
sees should be nothing. 
Theaet. No. How could that be ? 
Socr. Yet surely if what he sees is something, it must be 
a thing that is. Or do you suppose that ' something ' x 
can be reckoned among things that have no being at all ? 
Theaet. No, I don't. 

Socr. Then, if he sees something, he sees a thing that is. 
Theaet. Evidently. 
189. Socr. And if he hears a thing, he hears something and 
hears a thing that is. 
Theaet. Yes. 

1 The Greek els yi ns, ' at least some one ', is the contradictory of ovScts, 

not even one ', ' no one '. Zv yi ™ means ' a {= one) thing ' (ein Ding, une 

chose), as the opposite of ' no-thing ' ; and to Zv here means ' what is one ' 

(or ' a thing ' in this sense), while to Is /xt) ovoiv means the opposite, ' nothings \ 



189. Socr. And if he touches a thing, he touches something, 
and if something, then a thing that is. 
Theaet. That also is true. 

Socr. And if he thinks, 1 he thinks something, doesn't he ? 
Theaet. Necessarily. 

Socr. And when he thinks something, he thinks a thing 
that is ? 

Theaet. I agree. 

Socr. So to think what is not is to think nothing. 
Theaet. Clearly. 

Socr. But surely to think nothing is the same as not to 
think at all. 

Theaet. That seems plain. 
b. Socr. If so, it is impossible to think what is not, either 
about anything that is, or absolutely. 
Theaet. Evidently. 

Socr. Then thinking falsely must be something different 
from thinking what is not. 
Theaet. So it seems. 

Socr. False judgment, then, is no more possible for us on 
these lines than on those we were following just now. 
Theaet. No, it certainly is not. 

The problem developed in this argument is not a mere sophistic 
paradox, but a very real problem that is still being discussed. 
It will recur in the Sophist, where Plato, having brought the Forms 
upon the scene, will be able to offer a solution. 2 The statement of it 
is attributed to Protagoras elsewhere 3 : to think what is false is 
to think what is not ; but that is to think nothing ; and that, again, 
is not to think at all : therefore we can only think the thing that is, 
and all judgments must be true. Such was Protagoras' conclusion. 
Plato's is different, namely that, since there is such a thing as 
thinking falsely, it cannot be ' thinking what is not ', if that means 
(as the argument implies) having nothing at all before the mind. 
But the real significance of ' thinking what is not ' cannot be fol- 
lowed up here. It would involve drawing the necessary distinctions 
between various meanings of the terms ' is ' and ' is not ', and a 
discussion of the whole question of reality and unreality. All this 
is reserved for the Sophist, where the inquiry will start again from 
the problem as stated here, and follow the only line that can lead 
to a satisfactory conclusion. 

1 Or ' makes a judgment '. ' Thinks something ', again, is not distinguished 
from ' thinks of something '. 

2 See pp. 212 and 299 ff. 8 Euthydemus 286c and 283E. 


THEAETETUS 189b-190e 

Since the limits of the Theaetetus exclude a discussion of reality, 
the present argument has to be left where it is, and the transition to 
Socrates' next suggestion seems somewhat abrupt. We may, 
however, find a link, if we observe that the terms in which the debate 
had been carried on were too simple. Protagoras has been repre- 
sented earlier (167A) as asserting that ' one cannot think anything 
but what one experiences, and all experiences are true \ He saw 
no important distinction between what appears real to me in direct 
perception and what appears true to me, what I believe or judge 
to be true. ' Appears ' covered both. So he assumed that belief 
was like direct acquaintance with a sense-object, and must be 
infallible in the same way. What I believe, what I have before 
my mind when I think, must be something ; so there must be 
just that object or fact ; and there are no false facts, any more than 
non-existent objects. 

To escape this conclusion, further analysis is needed to bring 
out the distinction between direct acquaintance with sense-objects 
(which Plato has admitted to be infallible) and the process of making 
a judgment, which is not so simple and immediate as seeing a 
colour. It will be indicated that judgments of the type so far con- 
sidered — thinking that one thing is another thing — involve two 
terms, not to mention the connecting term ' is \ The act of making a 
judgment is not the same thing as perceiving this whole complex — 
perceiving a fact as we perceive a colour — but involves an operation 
of the mind which puts the terms together in a certain way. There 
may be room for mistakes to occur in this process, the nature of 
which Socrates will attempt to bring out gradually and to illustrate 
by images. 

189B-190E. The apparent impossibility of false judgment as mistak- 
ing one thing for another 

Socrates now recurs to the conception of false judgment as mis- 
taking one thing for another, or thinking that one thing is another. 
We are to examine what this can mean and in what circumstances 
it can occur. Our first conclusion (188c) that it was impossible 
resulted from the assumption that we must either ' know ' a thing 
(be acquainted with it and have it clearly before our minds) or not 
know it (be totally unacquainted with it). This dilemma does not 
really exhaust the possibilities. By taking memory into account, 
we can find a sense in which an object can be both known and not 

189B. Socr. Well, does the thing we call false judgment arise 
in this way ? 
Theaet. How ? 



189B. Socr. We do recognise the existence of false judgment as 
a sort of mis judgment, 1 that occurs when a person inter- 

c. changes in his mind two things, both of which are, and 
asserts that the one is the other. In this way he is always 
thinking of something which is, but of one thing in place 
of another, and since he misses the mark he may fairly be 
said to be judging falsely. 

Theaet. I believe you have got it quite right now. When 

a person thinks ' ugly ' in place of ' beautiful ' or ' beautiful ' 

in place of ' ugly ', he is really and truly thinking what is 


Socr. I can see that you are no longer in awe of me, 

Theaetetus, but beginning to despise me. 

Theaet. Why, precisely ? 

Socr. I believe you think I shall miss the opening you give 

me by speaking of ' truly thinking what is false ', and not 

d. ask you whether a thing can be slowly quick or heavily light 
or whether any contrary can desert its own nature and 
behave like its opposite. However, I will justify your bold- 
ness by letting that pass. So you like this notion that false 
judgment is mistaking. 

Theaet. I do. 

Theaetetus* phrase ' thinking (or judging) " ugly " in place of 
" beautiful " ' is vague and ambiguous. We should expect it to 
mean : thinking that some object which is in fact beautiful is ugly, 
or (in the language of later logic) assigning a wrong predicate to a 
subject. But this is not the sense taken in the following context. 
A discussion of what we call ' predicates ' would inevitably lead to 
the Forms. Possibly Theaetetus* remark is intended to remind us 
of their existence ; but Socrates will not bring them in. The 
field is still limited to judgments asserting that one (individual) 
thing is (identical with) another, as when I mistake Theaetetus 
for Socrates. 2 We are to consider how and when such a mistake 
can be made. 

189D. Socr. According to you, then, it is possible for the mind 
to take one thing for another, and not for itself. 
Theaet. Yes, it is. 

1 Plato coins a word dAAoSofm, ' misjudgment ', analogous to aXXoyvoelv, 
meaning to mistake one person for another. 

a Accordingly this hypothesis that false judgment is ' mistaking ' must 
not be confused with Plato's own analysis in the Sophist, which depends on 
the recognition of Forms. See p. 317. 


THEAETETUS 189b-190e 

189E. Socr. And when the mind does that, must it not be think- 
ing either of both things or of one of the two ? 
Theaet. Certainly it must, either at the same time or one 
after the other. 

Socr. Excellent. And do you accept my description of the 
process of thinking ? 
Theaet. How do you describe it ? 

Socr. As a discourse that the mind carries on with itself 
about any subject it is considering. You must take this 
explanation as coming from an ignoramus ; but I have a 
notion that, when the mind is thinking, it is simply talking 
to itself, asking questions and answering them, and saying 

190. Yes or No. When it reaches a decision — which may come 
slowly or in a sudden rush — when doubt is over and the two 
voices affirm the same thing, then we call that its ' judg- 
ment \ So I should describe thinking as discourse, and 
judgment as a statement pronounced, not aloud to someone 
else, but silently to oneself. 1 
Theaet. I agree. 

Socr. It seems, then, that when a person thinks of one 
thing as another, he is affirming to himself that the one is 
the other. 
b. Theaet. Of course. 

The effect of this account of thinking and judgment is to equate 
the act of ' mistaking ' one thing for another (' mis judgment ', the 
suggested equivalent of false judgment) with making the silent 
statement (Xoyog) that one thing is the other. So TheaetetuV phrase 
' judging " ugly " in place of " beautiful " * is reduced to making 
the statement that the beautiful (or what is beautiful) is ugly, or 
is the same thing as the ugly. 2 We are still considering only judg- 
ments of this type, which assert that one thing is another thing. 
We are supposed to have both things clearly before our minds 
(memory not having yet come into the discussion). Socrates pro- 
ceeds to point out that, within the limits of these assumptions, 
we never do judge that one thing is another. 

190B. Socr. Now search your memory and see if you have ever 
said to yourself ' Certainly, what is beautiful is ugly ', or 

1 This account of the process of thinking and judgment is repeated in the 
Sophist (see p. 318). 

2 Since the Forms are excluded from discussion, this expression ' the 
beautiful ' is left ambiguous. It can mean (1) anything that is beautiful 
(and recognised as such at the moment), or (2) Beauty itself (the Form). 
The ambiguity does not matter, because we never judge either that what we 
now see to be beautiful is ugly or that Beauty itself is Ugliness. 



190B. ' what is unjust is just \ To put it generally, consider if you 
have ever set about convincing yourself that any one thing 
is certainly another thing, or whether, on the contrary, you 
have never, even in a dream, gone so far as to say to yourself 
that odd numbers must be even, or anything of that sort. 
Theaet. That is true. 

c. Socr. Do you suppose anyone else, mad or sane, ever goes 
so far as to talk himself over, in his own mind, into stating 
seriously that an ox must be a horse or that two must be one ? 
Theaet. Certainly not. 

Socr. So, if making a statement to oneself is the same as 
judging, then, so long as a man is making a statement or 
judgment about both things at once and his mind has hold 
of both, he cannot say or judge that one of them is the 

D. other. You, in your turn, must not cavil at my language * ; 
I mean it in the sense that no one thinks : ' the ugly is 
beautiful ' or anything of that kind. 
Theaet. I will not cavil, Socrates. I agree with you. 
Socr. So long, then, as a person is thinking of both, he 
cannot think of the one as the other. 
Theaet. So it appears. 

Socr. On the other hand, if he is thinking of one only and 
not of the other at all, 2 he will never think that the one is 
the other. 

Theaet. True ; for then he would have to have before his 
mind the thing he was not thinking of. 
Socr. It follows, then, that ' mistaking ' is impossible, 
whether he thinks of both things or of one only. So 

e. there will be no sense in defining false judgment as ' mis- 

1 Burnet's text. In Greek ' the one ' and ' the other ' happen to be 
expressed by the same word, erepov. Socrates means : ' You must not cavil 
at my saying no one thinks one thing (to erepov) is another (erepov), on the 
verbal ground that erepov is the same word as erepov. I mean all the particular 
cases (such as ' the ugly is beautiful ') covered by this general formula.' 
The words inl ratv iv fxepei, eTreiBrj to [prjfia] erepov t<£ irepcp Kara prjfia ravrov iartv 
(B) may be a gloss on rrjSe, inserted in the wrong place ; or, if retained where 
they stand, they must mean ' You must let this phrase pass as applied to 
the particular cases (covered by them) ; for verbally the word erepov (one) is 
the same as the word erepov (other).' Cf. Madvig, Adv. Crit. i (1871), 377; 
Peipers, Erkenntnisstheorie Plato's 694. The koI aol (with eareov) means : 
You must pass my expression as I passed your ' a\r)0a>s ifrevSovs ' (at 189D). 

2 This sentence shows clearly that So£d£eiv (with accus.) here, as in other 
places in the context, means 'thinking of a thing, not making a judgment 
about it ; though ho£aoei in the next line does mean making the judgment 
that the one is the other. This is a good example of Plato's deliberate refusal 
to use terms as fixed technicalities. 


THEAETETUS 190e-195b 

190E. judgment \ It does not appear that false judgment exists in 
us in this form any more than in those we dismissed earlier. 
Theaet. So it seems. 

The upshot, so far, is that the notion of mistaking or interchanging 
one object for another will not explain how we can make a false 
judgment, so long as it is assumed that the objects must either be 
' known ' (clearly present to the mind) or else ' unknown ' (com- 
pletely absent from the mind). 

190E-195B. One class of mistakes can be explained by taking into 
account memory. The Wax Tablet 

The notion of ' mistaking ', however, need not be abandoned, if 
the assumption can be evaded ; and it can be evaded by introducing 
what has hitherto been excluded — the contents of the memory. 
We shall find that there is one class of false judgments that can be 
described as ' mistaking \ These are judgments in which the two 
things wrongly identified are objects of different sorts — one a present 
object of perception, the other a memory-image. So the scope of 
the discussion is now enlarged to include memory. 

190E. Socr. And yet, Theaetetus, if we cannot show that false 
judgment does exist, we shall be driven into admitting all 
sorts of absurdities. 
Theaet. For instance ? 

Socr. I will not mention them until I have tried to look 
at the question from every quarter. So long as we cannot 
see our way, I should feel some shame at our being forced 

191. into such admissions. But if we find the way out, then, 
as soon as we are clear, it will be time to speak of others as 
caught in the ludicrous position we shall have ourselves 
escaped ; though, if we are completely baffled, then, I sup- 
pose, we must be humble and let the argument do with us 
what it will, like a sailor trampling over sea-sick passengers. 
So let me tell you where I still see an avenue open for us to 

Theaet. Do tell me. 

Socr. I shall say we were wrong to agree that a man cannot 
think that things he knows are things he does not know and 
B. so be deceived. In a way it is possible. 

Theaet. Do you mean something that crossed my mind 
at the moment when we said that was impossible ? It 
occurred to me that sometimes I, who am acquainted with 
Socrates, imagine that a stranger whom I see at a distance is 



191B. the Socrates whom I know. In a case like that a mistake of 
the kind you describe does occur. 

Socr. And we were shy of saying that, because it would 
have made us out as both knowing and not knowing what 
we know ? 
Theaet. Exactly. 

Socr. We must, in fact, put the case in a different way. 
Perhaps the barrier will yield somewhere, though it may 

c. defy our efforts. Anyhow, we are in such straits that we 
must turn every argument over and put it to the test. 
Now, is there anything in this ? Is it possible to become 
acquainted with something one did not know before ? 
Theaet. Surely. 

Socr. And the process can be repeated with one thing 

after another ? 

Theaet. Of course. 

Socr. Imagine, then, for the sake of argument, that our 

minds contain a block of wax, which in this or that individual 

may be larger or smaller, and composed of wax that is com- 

d. paratively pure or muddy, and harder in some, softer in 
others, and sometimes of just the right consistency. 
Theaet. Very well. 

Socr. Let us call it the gift of the Muses' mother, Memory, 
and say that whenever we wish to remember something we 
see or hear or conceive in our own minds, we hold this wax 
under the perceptions or ideas and imprint them on it as we 
might stamp the impression of a seal-ring. Whatever is 
so imprinted we remember and know so long as the image 
remains ; whatever is rubbed out or has not succeeded in 

e. leaving an impression we have forgotten and do not know. 
Theaet. So be it. 

The word ' know ' has now received a new meaning : I know 
a thing when I have had direct acquaintance with it and an image 
of it remains stored in my memory. This gives a fuller range of 
possibilities than we have so far had. I may know Socrates in this 
sense and yet fail to recognise or identify him when I see him ; and 
I may mistake a stranger whom I see at a distance for the Socrates 
whom I know. This possibility of ' mistaking ' was excluded in the 
earlier argument by the false assumption that I must either know 
Socrates, in the sense of clearly perceiving him or having the 
thought of him clearly before my mind, or else my mind must be 
a complete blank concerning him. 

It may be noted that ideas or notions (evvoiai) are spoken of as 


THEAETETUS 190e-195b 

stamped on the memory, as well as perceptions. An idea is some- 
thing we ' conceive in our own minds ' (avrol ivvorjacofjiEv), but do 
not perceive. Its nature and origin are left obscure ; but the 
mention of such objects prepares the way for our knowledge of 
numbers, which are not perceived but are treated as images stamped 
in the memory (195E). 

191E. Socr. Now take a man who knows things in this way, and 
is attending to something that he sees or hears. Is- there 
not here a possibility of his making a false judgment ? 
Theaet. How ? 

Socr. By thinking that things he knows are other things 
he knows, or sometimes things he does not know. We 
were wrong when we agreed earlier that this was impossible. 
Theaet. What do you think about it now ? 

Socrates' next speech (192A, i-c, 5) contains a list of all the cases 
in which it is impossible to mistake one thing for another. He 
takes all the possible combinations of two objects which are (a) 
known (and now remembered) or (b) unknown (completely), (c) now 
perceived or (d) not now perceived. The conclusion is that there 
are only three combinations in which mistaking is possible. The 
reader would find the same difficulty as Theaetetus in following 
the statement and may prefer a summary to a translation. It 
will be simplest to use ' an acquaintance ' to mean a person (or 
thing) whom I know and of whom I have a memory image now before 
my mind ; and ' a stranger ' to mean a person (or thing) with which 
I have never been acquainted at all, a total stranger. 

Mistake, then, is impossible in the following cases : 

(1) If neither object is now perceived, I cannot mistake an 
acquaintance for another acquaintance, or confuse him with a 
stranger, or confuse two strangers. (These cases will be illustrated 
by examples at 193A-B.) 

(2) If perception only is involved, I cannot confuse two things 
which I see, or an object seen with an object not seen, or two objects 
neither of which is seen. 

(3) Where both knowledge and perception are involved, I cannot 
confuse two acquaintances both now seen and recognised 1 ; or 
confuse an acquaintance now seen and recognised with an absent 
acquaintance or with a stranger who is present. And there can be 
no confusion of two total strangers, whether I now see one of them 
or not. 

1 To recognise is to fit the new perception to the right memory-image, left 
by a former perception of the same object. 



Socrates now gives a summary statement of the three cases where 
mistake is possible, and these are illustrated in detail. 

192c, 5. Socr. (continues). There remain, then, the following cases 
in which, if anywhere, false judgment can occur. 
Theaet. What are they ? Perhaps they may help me to 
understand better. At present I cannot follow. 
Socr. Take things you know : you can suppose them to be 
other things which you both know and perceive ; or to be 
things you do not know, but do perceive ; or you can confuse 
D. two things which you both know and perceive. 
Theaet. Now I am more in the dark than ever. 
Socr. Let me start again, then, and put it in this way. I 
know Theodorus and have a memory in my mind of what he 
is like, and the same with Theaetetus. At certain moments 
I see or touch or hear or otherwise perceive them ; at other 
times, though I have no perception of you and Theodorus, 
I nevertheless remember you both and have you before my 
mind. Isn't that so ? 
e. Theaet. Certainly. 

Socr. That, then, is the first point I want to make clear — 
that it is possible either to perceive or not to perceive 
something one is acquainted with. 
Theaet. True. 

Socr. And it is also possible, when one is not acquainted 
with a thing, sometimes not to perceive it either, sometimes 
merely to perceive it and nothing more. 
Theaet. That is possible too. 

Socrates now takes, for illustration, three cases from his list, 
where mistake is impossible. They are cases in which no present 
perception is involved. (1) When nothing is before my mind except 
images of things I have formerly become acquainted with, I cannot 
judge that one of these remembered things is the other. (2) If I 
have an image of one only, I cannot judge that the thing is something 
I have never known. (3) Still less can I identify or confuse two 
things, neither of which I have ever known. 

192E. Socr. Then see if you can follow me better now. If 
193. Socrates knows Theodorus and Theaetetus, but sees neither 

and has no sort of present perception of them, he can never 

think in his own mind that Theaetetus is Theodorus. Is 

that good sense ? 

Theaet. Yes, that is true. 

Socr. Well, that was the first of the cases I mentioned. 


THEAETETUS 190e-195b 

193. Theaet. Yes. 

Socr. And the second was this : if I know one of you but 
not the other and perceive neither, once more I could never 
think that the one I know is the other whom I do not know. 
Theaet. True. 

b. Socr. And thirdly, if I neither know nor perceive either 
of you, I cannot think that one unknown person is another 
unknown person. And now take it as if I had gone over 
the whole list of cases again, in which I shall never judge 
falsely about you and Theodorus, whether I know both or 
neither or only one of you. And the same applies to perceiv- 
ing, if you follow me. 

Theaet. I follow now. 

1 The same applies to perceiving • refers to the second class of 
cases, where perception only is involved. If there is nothing but 
two objects of perception, you cannot mistake the one for the 
other, whether you perceive both or neither or one only. There 
remains the third class of cases, where both previous acquaintance 
and present perception are concerned. Among these Socrates now 
illustrates the three cases in which mistake is possible. 

193B. Socr. It remains, then, that false judgment should occur 
in a case like this : when I, who know you and Theodorus 

c. and possess imprints of you both like seal-impressions in 
the waxen block, see you both at a distance indistinctly 
and am in a hurry to assign the proper imprint of each 
to the proper visual perception, like fitting a foot into its 
own footmark to effect a recognition 1 ; and then make 
the mistake of interchanging them, like a man who thrusts 
his feet into the wrong shoes, and apply the perception of 
each to the imprint of the other. Or my mistake might 
be illustrated by the sort of thing that happens in a mirror 

D. when the visual current transposes right to left. 2 In that 
case mistaking or false judgment does result. 
Theaet. I think it does, Socrates. That is an admirable 
description of what happens to judgment. 
Socr. Then there is also the case where I know both 
and perceive only one, and do not get the knowledge I 

1 An allusion to the recognition of Orestes by his footmark tallying with 
his sister Electra's, Aeschylus, Choephori, 205 ff. 

2 Plato explains reflection by supposing that a stream of light (the visual 
current) from the eye coalesces at the surface of the mirror with a stream of 
light (colour) from the object. How the transposition occurs will be explained 
below, p. 327. 



193D. have of that one to correspond with my perception. That 
is the expression I used before, which you did not under- 
Theaet. No, I did not. 

The first of these two cases might be called the mistake of double 
transposition. The second is really similar, but simpler, involving 
only a single transposition of the same type. Instead of two false 
judgments : ' Yonder man (Theodorus) is Theaetetus, and that 
other man (Theaetetus) is Theodorus ', we now have only one. 
There is also the third case (192c) where I mistake a stranger 
whom I see for someone I remember. This is of the same pattern : 
I wrongly identify something now perceived (whether formerly 
known or not known, does not matter) with something I know. 
Socrates does not illustrate this, but now repeats his explanation 
of the two cases he has illustrated. * 

193D. Socr. Well, that is what I was saying : if you know 
e. one of two people and also perceive him and if you get the 
knowledge you have to correspond with the perception of 
him, you will never think he is another person whom you 
both know and perceive, if your knowledge of him likewise 
is got to correspond with the perception. That was so, 
wasn't it ? 
Theaet. Yes. 

Socr. But there was left over the case I have been describ- 
ing now, in which we say false judgment does occur : the 
possibility that you may know both and see or otherwise 

194. perceive both, but not get the two imprints to correspond 
each with its proper perception. Like a bad archer, you 
may shoot to one side and miss the mark — which is indeed 
another phrase we use for error. 
Theaet. With good reason. 

Socr. Also, when a perception is present which belongs 
to one of the imprints, but none which belongs to the 
other, and the mind fits to the present perception the im- 
print belonging to the absent one, in all such cases it is 
in error. To sum up : in the case of objects one does not 
B. know and has never perceived, there is, it seems, no possi- 
bility of error or false judgment, if our present accourt 
is sound ; but it is precisely in the field of objects both 
known and perceived that judgment turns and twists about 
and proves false or true — true when it brings impressions 
straight to their proper imprints ; false when it misdirects 
them crosswise to the wrong imprint. 


THEAETETUS 190e-195b 

194B. Theaet. Surely that is a satisfactory account, isn't it, 
Socrates ? 
c. Socr. You will think still better of it when you hear the 
rest. To judge truly is a fine thing and there is something 
discreditable in error. 
Theaet. Of course. 

Socr. Well, they say the differences arise in this way. 
When a man has in his mind a good thick slab of wax, 
smooth and kneaded to the right consistency, and the 
impressions that come through the senses are stamped on 
these tables of the ' heart ' — Homer's word hints at the 
D. mind's likeness to wax * — then the imprints are clear and 
deep enough to last a long time. Such people are quick 
to learn and also have good memories, and besides they 
do not interchange the imprints of their perceptions but 
think truly. These imprints being distinct and well- 
spaced are quickly assigned to their several stamps — the 
1 real things ' as they are called — and such men are said 
to be clever. Do you agree ? 
Theaet. Most emphatically. 
e. Socr. When a person has what the poet's wisdom com- 
mends as a ' shaggy heart ', or when the block is muddy 
or made of impure wax, or over soft or hard, the people 
with soft wax are quick to learn, but forgetful, those with 
hard wax the reverse. Where it is shaggy or rough, a 
gritty kind of stuff containing a lot of earth or dirt, the 
impressions obtained are indistinct ; so are they too when 
the stuff is hard, for they have no depth. Impressions in 

195. soft wax also are indistinct, because they melt together 
and soon become blurred. And if, besides this, they 
overlap through being crowded together into some wretched 
little narrow mind, they are still more indistinct. All 
these types, then, are likely to judge falsely. When they 
see or hear or think of something, they cannot quickly assign 
things to their several imprints. Because they are so slow 
and sort things into the wrong places, they constantly see 
and hear and think amiss, and we say they are mistaken 
about things and stupid. 

1 The Homeric word for heart (/ccap) resembles Krjpos (wax). Beare (Gk. 
Theories of Elem. Cognition 267) remarks that, had Plato chosen any physical 
organ to correspond to the wax as the seat of memory, it would probably 
have been the heart, the brain being the instrument of reason. There is 
no satisfactory evidence that the comparison of memory to a waxen block 
had ever been used before, except as a poet's metaphor (Aesch. P.V . 815 
liv-qfxocnv beXrois <f>p€va>v, Eutn. 275, etc.). 



195 b. Theaet. Your description could not be better, Socrates. 
Socr. We are to conclude, then, that false judgments do 
exist in us ? 

Theaet. Most certainly. 
Socr. And true ones also, I suppose ? 
Theaet. True ones also. 

Socr. At last, then, we believe we have reached a satis- 
factory agreement that both these kinds of judgments 
certainly exist ? 
Theaet. Most emphatically. 

It does not appear that Plato offers his waxen block as anything 
more than an illustration, a mechanical model which helps us to 
distinguish a memory-image from a fresh impression of sense, and 
to imagine the process of fitting the one to the other correctly 
or incorrectly. The conclusion, that true and false judgments of 
this type do exist, rests simply on familiar experience. The illus- 
tration serves to bring out the point that error comes in, not in 
the act of direct perception, but in judgments we make about 
what we perceive. This is an advance on Protagoras, who drew 
no distinction between what ' appears ' to me to be true (what 
I believe or think) and what ' appears ' to me as real in perception. 
But his account of false judgment as ' thinking the thing that is 
not ' and his denial that such a thing is possible have been shelved. 
This thesis is reserved for the Sophist. 

195B-196C False judgment in general cannot, however, be defined 
as the misfitting of perception to thought 

The weak point, however, is this. Only a small class of false 
judgments, even about things we now perceive, consist in identify- 
ing them with things we formerly perceived and now remember. 
This is the only type of judgment so far considered and described. 
It has been agreed, as a matter of common experience, that such 
judgments do exist. But there is an immense class of judgments, 
true and false, about things I do not now perceive and never have 
perceived. All historical judgments about events outside my own 
experience belong to this class. There are also, as Socrates now 
observes, true and false judgments about things that never can 
be perceived. Hence all that has been established is that false 
judgment does exist in a very small class of cases where we wrongly 
identify something we perceive. This is important, as contradicting 
Protagoras' doctrine that false judgment is impossible. But it 
has now to be pointed out that this ' mistaking ' or wrong ' fitting 
together of thought and perception ' is not a definition of false 
judgment in general. It will not cover cases where no perception 


THEAETETUS 195b-196c 

is involved. We can make mistakes about numbers, which are 
not objects of perception but are said to be ' known ' in the sense 
we have just given to that term, i.e. registered as imprints in the 
memory. We must accordingly retract the earlier statement that 
mistakes cannot occur between two objects both known but not 

195B. Socr. It really does seem to be true, Theaetetus, that a 
garrulous person is a strange and disagreeable creature. 1 
Theaet. Why, what makes you say that ? 

c. Socr. Disgust at my own stupidity. I am indeed garru- 
lous : what else can you call a man who goes on bandying 
arguments to and fro because he is such a dolt that he 
cannot make up his mind and is loath to surrender any one 
of them ? 

Theaet. But why are you disgusted with yourself ? 
Socr. I am not merely disgusted but anxious about the 
answer I shall make if someone asks : ' So, Socrates, you 
have made a discovery : that false judgment resides, not 
in our perceptions among themselves nor yet in our thoughts, 

d. but in the fitting together of perception and thought ? ' 
I suppose I shall say, Yes, and plume myself on this brilliant 
discovery of ours. 

Theaet. I don't see anything to be ashamed of in what 
you have just pointed out, Socrates. 

Socr. ' On the other hand/ he will continue, ' you also 
say that we can never imagine that a man whom we merely 
think of and do not see is a horse which again we do not 
see or touch but merely think of without perceiving it 
in any way ? ' I suppose I shall say, Yes, to that. 
Theaet. And rightly. 

e. Socr. ' On that showing/ he will say, ' a man could never 
imagine that 11, which he merely thinks of, is 12, which 
again he merely thinks of.' Come, you must find the answer 

Theaet. Well, I shall answer that, if he saw or handled 
eleven things, he might suppose they were twelve ; but 
he will never make that judgment about the 11 and the 
12 he has in his thoughts. 

Socr. Well now, does a man ever consider in his own 

196. mind 5 and 7 — I don't mean five men and seven men or 

anything of that sort, but just 5 and 7 themselves, which 

1 ' Garrulity ' or ' babbling ' was an abusive term applied to the conversa- 
tions of Socrates and his associates. See below, p. 176, on Soph. 225D. 



196. we describe as records in that waxen block of ours, among 
which there can be no false judgment — does anyone ever 
take these into consideration and ask himself in his inward 
conversation how much they amount to ; and does one 
man believe and state that they make 11, another that 
they make 12, or does everybody agree they make 12 ? 

b. Theaet. Far from it ; many people say n ; and if larger 
numbers are involved, the more room there is for mistakes ; 
for you are speaking generally of any numbers, I suppose. 
Socr. Yes, that is right. Now consider what happens 
in this case. Is it not thinking that the 12 itself that is 
stamped on the waxen block is 11 ? 

Theaet. It seems so. 

Socr. Then haven't we come round again to our first 
argument ? For when this happens to someone, he is 
thinking that one thing he knows is another thing he 
knows ; and that, we said, was impossible. That was the 
very ground on which we were led to make out that there 
could be no such thing as false judgment : it was in order 

c. to avoid the conclusion that the same man must at the 
same time know and not know the same thing. 
Theaet. Quite true. 

Socr. If so, we must account for false judgment in some 
other way than as the misfitting of thought to perception. 
If it were that, we should never make mistakes among 
our thoughts themselves. As the case stands now, either 
there is no such thing as false judgment, or it is possible 
not to know what one does know. Which alternative do 
you choose ? 
Theaet. I see no possible choice, Socrates. 

The Platonist may here be surprised to find our knowledge of 
a number regarded as the record in the memory-tablet of an im- 
pression, as if we became acquainted with the number 12 in the 
same way as with a colour or a sound or a person. Has Plato 
abandoned his doctrine of Recollection, according to which our 
knowledge of Forms, including numbers and their relations, is 
always latent in the soul, not acquired through the senses during 
this life, but only revived on the occasion of sense-experience ? 
There is no ground for such a conclusion. The whole dialogue 
examines the claim of the world of external sensible objects to 
be the sole source of knowledge. This claim is taken as implying 
that outside us there are physical objects which can yield us sense- 
data through the several organs, and inside us a tabula rasa on 


THEAETETUS 196d-199c 

which impressions so received can be stamped and recorded. This 
mechanism is based on the empiricist assumption that all our know- 
ledge must be derived somehow from the external objects of per- 
ception. On this assumption (which Plato himself does not accept) 
our idea of the number 12 must be supposed to be extracted from 
a series of sense-impressions and added to our memory records. 
As Campbell remarks, ' memory is made to do the work of abstrac- 
tion \ This is all the apparatus that has so far come into view. 
It has sufficed to illustrate one class of mistakes — the wrong fitting- 
together of old records and new impressions. But we have now 
seen that this formula will not cover the mistaking of one memory 
record for another, and so it will not do as a general account of 
false judgment. We cannot admit mistakes about numbers, unless 
we can find a sense in which we can not know something we do 
know. The empiricist's apparatus will have to be enlarged. 

196D-199C. Memory compared to an aviary, to provide for mistaken 
judgments not involving perception 

Objection might be taken to the statement (196B) that, when 
we make the mistake, we ' think that the 12 on our wax-tablet is 
11 ', or that ' one thing we know (12) is another thing we know 
(11) \ It is still presumed that a false judgment must consist in 
wrongly identifying one thing with another. Even if that were 
so, what we identify with 11 is, not 12, but ' the sum of 5 and 7 ' 
— a number which at the moment we do not know (in a sense). 
We are wondering what number it is, and wrongly conclude that 
it is 11. The number 12, although we are familiar with it, is not 
present to our mind. We do not judge that 12 is 11. 

This objection, it is true, does not invalidate the only conclusion 
stated : that the misfitting of thought and perception cannot be 
a definition of false judgment in general. But it serves to bring 
out the need for some enlargement of the empiricist apparatus — 
some further distinction between the meanings of the word ' know '. 
The misleading statement that ' we judge the 12 in our waxen 
block to be 11 ' is a consequence of the too narrow use of ' know ' 
in terms of that image. To ' know ' meant to have become ac- 
quainted with a thing and to ' remember ' it in the sense of having 
the memory of it now before the mind. If I remember both 11 
and 12 in that way, to confuse them is as impossible as we said 
it was to confuse two absent friends when I now remember them 
both. Socrates, accordingly, goes on to distinguish yet another 
sense of ' know \ The image of an object may be registered in 
the memory without being present to our consciousness. It is 
possible not to know (have before our minds) what we do know 



(possess somewhere registered in memory). A new simile, the 
aviary, is now substituted for the waxen block to provide for this 
latent knowledge. We shall no longer need to speak as if the 
number 12 were present to our minds and confused with 11. 

196D. Socr. But the argument is not going to allow both alterna- 
tives. However, we must stick at nothing : suppose we 
try being quite shameless. 
Theaet. In what way ? 

Socr. By making up our minds to describe what knowing 
is like. 

Theaet. How is that shameless ? 

Socr. You seem to be unaware that our whole conversation 
from the outset has been an inquiry after the nature of know- 
ledge on the supposition that we did not know what it was. 
Theaet. No, I am quite aware of that. 
Socr. Then, doesn't it strike you as shameless to explain 
what knowing is like, when we don't know what knowledge 
e. is ? The truth is, Theaetetus, that for some time past 
there has been a vicious taint in our discussion. Times 
out of number we have said : ' we know ', 'we do not 
know ', 'we have knowledge ', ' we have no knowledge ', 
as if we could understand each other while we still know 
nothing about knowledge. At this very moment, if you 
please, we have once more used the words ' know nothing ' 
and ' understand ', as if we had a right to use them while 
we are still destitute of knowledge. 

Theaet. Well, but how are you going to carry on a dis- 
cussion, Socrates, if you keep clear of those words ? 

197. Socr. I cannot, being the man I am, though I might if 
I were an expert in debate. If such a person were here 
now, he would profess to keep clear of them and rebuke us 
severely for my use of language. As we are such bunglers, 
then, shall I be so bold as to describe what knowing is like ? 
I think it might help us. 

Theaet. Do so, then, by all means. And if you cannot 
avoid those words, you shall not be blamed. 
Socr. Well, you have heard what ' knowing ' is commonly 
said to be ? 

Theaet. Possibly ; but I don't remember at the moment. 
b. Socr. They say it is ' having knowledge \ 1 

1 This is of course not a ' definition ' of knowing, but a verbal paraphrase, 
which occurs at Euthyd. 277B. It may be due to Prodicus or some other 
writer on the correct use of language (nepl dvo/xarwv opdoT-qros). Prodicus is 
cited in the context at Euthyd. 277E. 


THEAETETUS 196d-199c 

197B. Theaet. True. 

Socr. Let us make a slight amendment and say : ' possess- 
ing knowledge \ 

Theaet. What difference would you say that makes ? 
Socr. None, perhaps ; but let me tell you my idea and 
you shall help me test it. 
Theaet. I will if I can. 

Socr. ' Having ' seems to me different from ' possessing \ 
If a man has bought a coat and owns it, but is not wearing 
it, we should say he possesses it without having it about 
him. 1 
Theaet. True. 

c. Socr. Now consider whether knowledge is a thing you 
can possess in that way without having it about you, like 
a man who has caught some wild birds — pigeons or what 
not — and keeps them in an aviary he has made for them 
at home. In a sense, of course, we might say he ' has ' 
them all the time inasmuch as he possesses them, mightn't 
we ? 

Theaet. Yes. 

Socr. But in another sense he ' has ' none of them, though 
he has got control of them, now that he has made them 
captive in an enclosure of his own ; he can take and have 
hold of them whenever he likes by catching any bird he 

D. chooses, and let them go again ; and it is open to him to 
do that as often as he pleases. 
Theaet. That is so. 

Socr. Once more then, just as a while ago we imagined 
a sort of waxen block in our minds, so now let us suppose 
that every mind contains a kind of aviary stocked with 
birds of every sort, some in flocks apart from the rest, some 
in small groups, and some solitary, flying in any direction 
among them all. 2 

e. Theaet. Be it so. What follows ? 

Socr. When we are babies we must suppose this recep- 
tacle empty, and take the birds to stand for pieces of 
knowledge. Whenever a person acquires any piece of 

1 "J5x«tvis commonly used of ' wearing ' a garment. It also means ' to have 
hold of ' — the phrase used below for holding the bird that has been caught 
inside the aviary. 

a Some classification of the objects of knowledge seems to be hinted at. 
Comparison with the Sophist (252E ff.) may suggest that the large and small 
groups of birds are generic and specific Forms, the solitary birds which fly 
among all the rest, Forms of universal application like Existence, Sameness, 
Difference. But nothing turns on such conjectures. 



197E. knowledge and shuts it up in his enclosure, we must say 
he has learnt or discovered the thing of which this is the 
knowledge, and that is what ' knowing ' means. 
Theaet. Be it so. 

198. Socr. Now think of him hunting once more for any piece 
of knowledge that he wants, catching and holding it, and 
letting it go again. In what terms are we to describe that 
— the same that we used of the original process of acquisi- 
tion, or different ones ? An illustration may help you to 
see what I mean. There is a science you call ' arithmetic '. 
Theaet. Yes. 

Socr. Conceive that, then, as a chase after pieces of know- 
, ledge about all the numbers, odd or even. 
Theaet. I will. 

Socr. That, I take it, is the science in virtue of which 
B. a man has in his control pieces of knowledge about numbers 
and can hand them over to someone else. 
Theaet. Yes. 

Socr. And when he hands them over, we call it ' teaching ', 
and when the other takes them from him, that is ' learning ', 
and when he has them in the sense of possessing them in 
that aviary of his, that is ' knowing \ 
Theaet. Certainly. 

Socr. Now observe what follows. The finished arith- 
metician knows all numbers, doesn't he ? There is no 
number the knowledge of which is not in his mind. 
Theaet. Naturally. 

c. Socr. And such a person may sometimes count either 
the numbers themselves in his own head or some set of 
external things that have a number. 

Theaet. Of course. 

Socr. And by counting we shall mean simply trying to 

find out what some particular number amounts to ? 

Theaet. Yes. 

Socr. It appears, then, that the man who, as we admitted, 

knows every number, is trying to find out what he knows 

as if he had no knowledge of it. No doubt you sometimes 

hear puzzles of that sort debated. 

Theaet. Indeed I do. 

d. Socr. Well, our illustration from hunting pigeons and 
getting possession of them will enable us to explain that 
the hunting occurs in two ways : first, before you possess 
your pigeon in order to have possession of it ; secondly, 
after getting possession of it, in order to catch and hold 


THEAETETUS 196d-199c 

198D. in your hand what you have already possessed for some 
time. In the same way, if you have long possessed pieces 
of knowledge about things you have learnt and know, it 
is still possible to get to know the same things again, by 
the process of recovering the knowledge of some particular 
thing and getting hold of it. It is knowledge you have 
possessed for some time, but you had not got it handy in 
your mind. 
Theaet. True. 
e. Socr. That, then, was the drift of my question, what 
terms should be used to describe the arithmetician who 
sets about counting or the literate person who sets about 
reading ; because it seemed as if, in such a case, the man 
was setting about learning again from himself what he 
already knew. 

Theaet. That sounds odd, Socrates. 
Socr. Well, but can we say he is going to read or count 

199. something he does not know, when we have already granted 
that he knows all the letters or all the numbers ? 
Theaet. No, that is absurd too. 

Socr. Shall we say, then, that we care nothing about 
words, if it amuses anyone to turn and twist the expressions 
1 knowing ' and ' learning ' ? Having drawn a distinction 
between possessing knowledge and having it about one, 
we agree that it is impossible not to possess what one does 
possess, and so we avoid the result that a man should not 
know what he does know ; but we say that it is possible 
for him to get hold of a false judgment about it. For 
b. he may not have about him the knowledge of that thing, 
but a different piece of knowledge instead, if it so happens 
that, in hunting for some particular piece of knowledge, 
among those that are fluttering about, he misses it and 
catches hold of a different one. In that case, you see, he 
mistakes 11 for 12, 1 because he has caught hold of the 
knowledge of 11 that is inside him, instead of his knowledge 
of 12, as he might catch a dove in place of a pigeon. 
Theaet. That seems reasonable. 

Socr. Whereas, when he catches the piece of knowledge 
he is trying to catch, he is not mistaken but thinks what 

1 Literally ' thinks 11 is 12 '. This cannot now mean that he has both 
numbers before his mind and judges one of them to be the other. This was 
agreed to be impossible (195E). It means that he mistakes the number 11, 
which he lays hold of for the number 12 which he was really looking for, 
when he asked : What is the sum of 7 and 5 ? 



199B. is true. In this way both true and false judgments can 
c. exist, and the obstacles that were troubling us are removed. 
You will agree to this, perhaps ? Or will you not ? 
Theaet. I will. 

Socr. Yes ; for now we are rid of the contradiction about 
people not knowing what they do know. That no longer 
implies our not possessing what we do possess, whether 
we are mistaken about something or not. 

The aviary has enlarged the machinery of the waxen block by 
providing for the process of hunting out latent pieces of knowledge 
and bringing them before the mind. So it has led to the suggestion 
that false judgment occurs when we get hold of the wrong piece 
of knowledge and ' interchange ' it for the right one. An important 
difference between the two images is that the process of originally 
acquiring knowledge is differently conceived. The waxen block 
was thought of as a receptacle for sense-impressions which left 
their imprint as memory-images. It seemed hard to imagine how 
one such imprint should ever be mistaken for another ; and no 
provision was made for historical knowledge or any knowledge 
not immediately derived from the senses. The aviary, on the other 
hand, represents knowledge as acquired from a teacher who ' hands 
over ' pieces of information to the learner. Such information would 
not consist in a series of separate imprints, but rather of statements 
offered for our belief. It would cover historical and abstract 
knowledge, as well as our notions of such things as numbers. 

Now, from the Meno onwards, Plato has repeatedly declared 
that what he calls ' knowledge ' is not a thing that can be ' handed 
over ' by one person to another. The true objects of knowledge 
must be directly seen by the eye of the soul ; the professors of 
education who claim to put into the mind knowledge that is not 
there are like one who should claim to put sight into blind eyes. 1 
The sophists are condemned for offering to ' hand over ' ' excellence ' 
(arete) of various sorts to their hearers. 2 In Plato's view all 
mathematical knowledge and knowledge of the Forms cannot, in 
the ordinary sense, be ' taught \ It is always in the soul and 
needs to be ' recollected '. The intervention of a teacher is not 
necessary, though the process may be directed and assisted by 
conversation (' dialectic ') with a wiser person who will act as 
midwife. The Platonist will see at once that what is here called 
a ' piece of knowledge ' can be nothing more than a belief (<5dfa), 
conveyed from one mind to another. All this cannot be openly 
said here, because the Forms are excluded from the discussion, 

1 Rep. 518c. 2 Meno 93B ; Euthyd. 273D, 287A. 


THEAETETUS 199c-200d 

which is confined to the empiricist claim that all knowledge comes 
from the external world of sense, either directly or by teaching 
as commonly conceived. But Plato is careful to note that we are 
still working on the empiricist assumption that the aviary is empty 
at birth — a tabula rasa — and gradually filled with contents derived 
from sensible experience and learning. The reader, guided by the 
long description of Socratic midwifery, is left to infer that these 
so-called ' pieces of knowledge ' are not knowledge at all. It is 
perhaps with intention that Plato, while describing the recovery 
of latent ' knowledge ', never uses his own word for recollection 

199C-200D. Rejection of ' interchange of pieces of knowledge ' as 
an account of False Judgment 

The aviary has enabled us to imagine how a man who has learnt 
that the sum of 7 and 5 is 12, may sometimes ask himself what 
the sum of 7 and 5 is, and get hold of a wrong ' piece of knowledge ', 
viz. the number n, which he is also acquainted with. He mistakes 
this for the * piece of knowledge ' he wants, namely 12. This 
1 interchange * may seem to be an unobjectionable description of 
such a mistake. Socrates, however, at once raises an objection, 
which turns upon the unexplained term ' piece of knowledge \ 

199c. Socr. {continues). But it strikes me that a still stranger 
consequence is coming in sight. 
Theaet. What is that ? 

Socr. That the interchange of pieces of knowledge should 
ever result in a judgment that is false. 
Theaet. How do you mean ? 
D. Socr. In the first place, that a man should have knowledge 
of something and at the same time fail to recognise x that 
very thing, not for want of knowing it but by reason of 
his own knowledge ; and next that he should judge that 
thing to be something else and vice versa — isn't that very 
unreasonable : that when a piece of knowledge presents 
itself, the mind should fail to recognise anything and know 
nothing ? On this showing, the presence of ignorance might 
just as well make us know something, or the presence of 
blindness make us see — if knowledge can ever make us fail 
to know. 

This objection is obscure, and the language ambiguous : ayvoelv 
can mean either ' to be ignorant of ' or ' to fail to recognise ' (the 

1 For ayvociv, meaning ' fail to recognise ', cf. i88b. 



opposite of yvcbvai, * to recognise '). The ' piece of knowledge 
that presents itself ' must mean the number n, which I have 
laid hold of instead of the number 12 which I was looking for 
and have not found. In what sense does the interchange involve 
that I should ' fail to recognise (ayvoelv) that very thing, not for 
want of knowing it (dyvcojuoovvr)) but by reason of my own know- 
ledge ' ? ' Fail to recognise that very thing ' (the number n) 
can only mean that I fail to recognise the fact that it is not the 
number I want ; hence Socrates says I judge it to be 12, i.e. mistake 
it for 12. But ' not for want of knowing it ' (ayvoyfjuoavvrj) means 
1 not for want of be ingacquainted with it \ The situation is analogous 
to what was described earlier : I see an acquaintance and, failing 
to recognise him, mistake him for another acquaintance. But 
there perception was involved, and the mistake was explained as 
the fitting-together of the fresh impression and the wrong memory- 
image. Here no perception is involved. Socrates 1 point seems 
to be that the aviary contains nothing but ' pieces of knowledge \ 

I am acquainted with both the numbers, 11 and 12. One of them 
(11) is now before my mind. How can I mistake that number 
for the other which I am also acquainted with ? If I have been 
taught and know the truth that 7 + 5 = 12, how can I substitute 

II for 12 and believe that I have got hold of the right number ? 
There is no question here of seeing something dimly at a distance ; 
only ' pieces of knowledge ' are involved. 

To this we might reply that an analogous explanation by the 
misfitting of two pieces of knowledge could be given, if the unex- 
plained term ' piece of knowledge ' were taken in a sufficiently 
wide sense. The expression covers objects (such as numbers) that 
I am acquainted with, as well as truths that I have been taught. 
All these are in my aviary. Does it also include a complex object 
such as ' the sum of 7 and 5 ' ? This ought to be included ; it 
consists of terms I am acquainted with and it is before my mind 
when I ask : what is the sum of 7 and 5 ? It is this object that 
I identify with 11 when I make my false judgment. If it is a 
1 piece of knowledge ' and contained in the aviary, then the false 
judgment can be explained as the wrong putting-together of two 
pieces of knowledge, as in the waxen block false judgment was the 
putting-together of a fresh impression and the wrong memory 
imprint. The result will be a false judgment entirely composed 
of ' pieces of knowledge ' (terms I am acquainted with). It thus 
seems that the aviary apparatus is, after all, as adequate to explain 
false judgment where no perception is involved as the waxen block 
was to explain false judgment involving perception. 

It is hard to resist the impression that Plato has overlooked this 


THEAETETUS 199c-200d 

explanation, because he does not recognise ' the sum of 7 and 5 ' 
as a ' piece of knowledge ', but persists in speaking as if we judged 
not that ' the sum of 7 and 5 is 11 ' but that ' 12 (the number we 
are seeking) is 11 (the number we lay hold of) '. If such objects 
as ' the sum of 7 and 5 ' are excluded, then the difficulty Socrates 
raises does exist : how can I mistake the 11 which I have before 
my mind for the 12 which I know but have not before my mind ? 
Theaetetus, at any rate, does not put forward the explanation 
above offered. He takes up Socrates' word for ' ignorance ' or 
1 failure to recognise ' (dyvcojuoavvrj) , and suggests that our minds 
may contain ' pieces of ignorance ' as well as ' pieces of knowledge \ 

199E. Theaet. Perhaps, Socrates, we were wrong in making the 
birds stand for pieces of knowledge only, and we ought to 
have imagined pieces of ignorance flying about with them 
in the mind. Then, in chasing them, our man would lay 
hold sometimes of a piece of knowledge, sometimes of a 
piece of ignorance ; and the ignorance would make him 
judge falsely, the knowledge truly, about the same thing. 

What is a ' piece of ignorance ' ? Evidently not an object I am 
unacquainted with, for then it would not be in the aviary at all. 
It can only be a false belief which I have somehow formed or been 
taught, such as that 7 + 5 = n. There is no reason why false 
beliefs should not be in the aviary ; in fact our aviaries contain only 
too many. In so far as they consist of terms I am acquainted with 
and are things that I have learnt and possess stored in my memory, 
they satisfy the description of ' pieces of knowledge '. But they are 
not knowledge in the sense in which whatever is knowledge must be 
true. That they are simply false beliefs is practically stated in 
Theaetetus' last words : * the ignorance would make him judge 
falsely '. Theaetetus' suggestion means that what I lay hold of is 
an old false belief which I bring up into consciousness. 

An obvious answer to Theaetetus' suggestion would be this : 
' You explain my making a false judgment now as my getting hold 
of an old false belief which I have acquired and have in my memory ; 
but that does not explain how I could acquire that false belief 
originally. You merely push back to an earlier stage the same 
problem : how could I ever judge that 7 + 5 = II ? ' Socrates, 
however, does not raise that objection. Taking Theaetetus' sugges- 
tion that I call up and affirm an old false belief, he asks how it is 
that I fail to recognise it as false and mistake it for a true piece of 

199E. Socr. It is not easy to disapprove of anything you say, 
Theaetetus ; but think again about your suggestion. Sup- 



199E. pose it is as you say ; then the man who lays hold of the 
200. piece of ignorance will judge falsely. Is that right ? 
Theaet. Yes. 

Socr. But of course he will not think he is judging falsely. 
Theaet. Of course not. 

Socr. No ; he will think he is judging truly ; and his 
attitude of mind will be the same as if he knew the thing 
he is mistaken about. 
Theaet. Naturally. 

Socr. So he will imagine that, as a result of his chase, he 
has got hold of a piece of knowledge, not a piece of ignorance. 
Theaet. Clearly. 

Socr. Then we have gone a long way round only to find 
ourselves confronted once more with our original difficulty. 
Our destructive critic will laugh at us. ' You wonderful 
B. people/ he will say, ' are we to understand that a man 
knows both a piece of knowledge and a piece of ignorance, 
and then supposes that one of these things he knows is the 
other which he also knows ? Or does he know neither, and 
then judge that one of these unknown things is the other ? 
Or does he know only one, and identify this known thing 
with the unknown one, or the unknown one with the known ? 
Or are you going to tell me that there are yet further pieces 
of knowledge about your pieces of knowledge and ignorance, 
and that their owner keeps these shut up in yet another of 
c. your ridiculous aviaries or waxen blocks, knowing them so 
long as he possesses them, although he may not have them 
at hand in his mind ? On that showing you will find your- 
selves perpetually driven round in a circle and never getting 
any further.' What are we to reply to that, Theaetetus ? 
Theaet. Really, Socrates, I don't know what we are to say. 
Socr. Maybe, my young friend, we have deserved this 
rebuke, and the argument shows that we were wrong to 
D. leave knowledge on one side and look first for an explanation 
of false judgment. That cannot be understood until we 
have a satisfactory account of the nature of knowledge. 
Theaet. As things now stand, Socrates, one cannot avoid 
that conclusion. 

The critic objects that it is as hard to explain how I can fail to 
recognise a false belief as false and mistake it for the true belief 
which I possess stored in my mind, as it is to explain how I can 
mistake an object before my mind for another object which is in 
my memory. As Socrates indicates, that leads on to the question : 


THEAETETUS 200d-201c 

How can I know that I know ? How can I recognise knowledge 
when I have it and be sure that it is knowledge ? This is an old 
problem inconclusively discussed in the Charmides (167 ff.). Plato 
refuses to pursue it here, or to carry any further the attempt to 
account for false belief. 

What has emerged is that the term ' knowledge ' is very 
ambiguous. Until we have discovered all its meanings, we cannot 
really explain false judgment. The discussion has been fruitful in 
bringing to light some of these meanings. But the scope of the 
dialogue excludes all that Plato calls knowledge in the full sense. 
He breaks off here because he cannot go further without invoking 
the true objects of knowledge. Plato's own analysis of false judg- 
ment will be given in the Sophist, when the Forms have been brought 
into view. 

200D-201C. Conclusion : Knowledge cannot be defined as True 

It has become clear that the so-called ' pieces of knowledge ' which 
I have learnt from a teacher and stored in my memory are nothing 
better than true beliefs. When I recall one to consciousness my 
attitude of mind towards it is, as Socrates says, indistinguishable 
from my attitude to a false belief. This consideration leads us to 
the next point : the final refutation of the claim of true belief to 
be knowledge. My confidence in a mere belief is not grounded in 
reason. The teaching which consists in ' handing over ' beliefs, 
whether true or false, is no better than the rhetorical persuasion 
of a barrister. Knowledge is not so gained ; and when it is gained, 
it cannot be shaken by persuasion. 

200D. Socr. To start all over again, then : what is one to say 
that knowledge is ? For surely we are not going to give 
up yet. 

Theaet. Not unless you do so. 

Socr. Then tell me : what definition can we give with 
the least risk of contradicting ourselves ? 
e. Theaet. The one we tried before, Socrates. I have noth- 
ing else to suggest. 
Socr. What was that ? 

Theaet. That true belief is knowledge. Surely there can 
at least be no mistake in believing what is true and the 
consequences are always satisfactory. 1 

1 It has been pointed out in the Meno (97A) that for practical purposes it 
is as useful to believe that a road leads to a certain place as to know that it 
does. Cf . also Rep. 506c : belief without knowledge is at the best like a blind 
man who takes the right road. 



200E. Socr. Try, and you will see, Theaetetus, as the man said 
when he was asked if the river was too deep to ford. So 
here, if we go forward on our search, we may stumble upon 

201. something that will reveal the thing we are looking for. We 
shall make nothing out, if we stay where we are. 
Theaet. True ; let us go forward and see. 
Socr. Well, we need not go far to see this much : you 
will find a whole profession to prove that true belief is not 

Theaet. How so ? What profession ? 
Socr. The profession of those paragons of intellect known 
as orators and lawyers. There you have men who use their 
skill to produce conviction, not by instruction, but by making 
people believe whatever they want them to believe. You 
B. can hardly imagine teachers so clever as to be able, in the 
short time allowed by the clock, to instruct their hearers 
thoroughly in the true facts of a case of robbery or other 
violence which those hearers had not witnessed. 
Theaet. No, I cannot imagine that ; but they can con- 
vince them. 

Socr. And by convincing you mean making them believe 
Theaet. Of course. 

Socr. And when a jury is rightly convinced of facts which 
can be known only by an eye-witness, then, judging by hear- 
c. say and accepting a true belief, they are judging without 
knowledge, although, if they find the right verdict, their 
conviction is correct ? 
Theaet. Certainly. 

Socr. But if true belief and knowledge were the same thing, 
the best of jurymen could never have a correct belief without 
knowledge. It now appears that they must be different 

This argument is repeated in a later dialogue, the Timaeus (5 id), 
where the existence of the Forms is said to follow from the distinc- 
tion between knowledge or rational understanding (vovg) and true 
belief. Knowledge is produced by instruction, always accompanied 
by a true account of its grounds (dXrjdrjg Xoyog), unshakable by 
persuasion, and possessed by gods and only a few among men. 
True belief is produced by persuasion, not based on rational grounds 
{aXoyov), can be changed by persuasion, and is possessed by all 

In our passage Socrates has not spoken of the absence of rational 


THEAETETUS 201c-202c 

grounds, such as he has in mind in the Meno and the Timaeus. 
In both those dialogues Plato is thinking of what he himself calls 
knowledge. In the Meno mathematical knowledge is in question. 
After his experiment with the slave, Socrates remarks that the 
slave has now a true belief about the solution ; but it will not be 
knowledge until he has been taken repeatedly through all the steps 
of the proof. He will then see for himself, with unshakable convic- 
tion, that the conclusion must be true. His belief will now be 
assured by reflection on the grounds or reasons (air (ag Xoyiofxcp). 
Such is the ' true account of the grounds ' (dXrjdrjg Xoyog) to which 
the Timaeus refers. But here the real objects of knowledge are 
not to be mentioned, and Socrates is only allowed his analogous 
contrast between the juryman's second-hand belief and the direct 
' knowledge ' of the eye-witness, who has seen the fact for himself. 

III. The claim of True Belief accompanied by an Account 
or Explanation to be Knowledge 

201C-202C. Socrates states this theory as he has heard it 

Theaetetus' next suggestion is that the addition of some kind of 
1 account ' or ' explanation ' (logos) * will convert true belief into 
knowledge. Various possible senses of ' account ' are distinguished 
and considered, and the suggestion is finally rejected. It will 
appear, however, that no one of these senses is the sense which 
' account ' bears in the Meno and the Timaeus. Why that sense 
is ignored will become clear as we proceed. 

201C Theaet. Yes, Socrates, I have heard someone make the 
distinction. 2 I had forgotten, but now it comes back to me. 
d. He said that true belief with the addition of an account 
(logos) was knowledge, while belief without an account was 
outside its range. Where no account could be given of a 
thing, it was not ' knowable ' — that was the word he used 
— where it could, it was knowable. 

Socr. A good suggestion. But tell me how he distinguished 
these knowable things from the unknowable. It may turn 
out that what you were told tallies with something I have 
heard said. 

1 English provides no single equivalent for logos, a word which covers 
(i) statement, speech ; (2) expression, definition, description, formula ; 
(3) ' tale ' or enumeration ; (4) explanation, account, ground. A translator 
is forced to use now one, now another of these expressions. In the text the 
word remains ambiguous until Socrates distinguishes some of its chief 

2 Between knowledge and true belief. 



201D. Theaet. I am not sure if I can recall that ; but I think 
I should recognise it if I heard it stated. 
Socr. If you have had a dream, let me tell you mine in 
return. I seem to have heard some people say that what 
E. might be called the first elements * of which we and all other 
things consist are such that no account can be given of them. 
Each of them just by itself can only be named ; we cannot 
attribute to it anything further or say that it exists or does 

202. not exist ; for we should at once be attaching to it existence 
or non-existence, whereas we ought to add nothing if we 
are to express just it alone. We ought not even to add 
1 just ' or ' it ' or ' each ' or ' alone ' or ' this ' 2 , or any other 
of a host of such terms. These terms, running loose about 
the place, are attached to everything, and they are distinct 
from the things to which they are applied. If it were possible 
for an element to be expressed in any formula exclusively 
belonging to it, no other terms ought to enter into that 
expression ; but in fact there is no formula in which any 

b. element can be expressed : it can only be named, for a name 
is all there is that belongs to it. But when we come to things 
composed of these elements, then, just as these things are 
complex, so the names are combined to make a description 
(logos), a description being precisely a combination of names. 
Accordingly, elements are inexplicable and Unknowable, but 
they can be perceived ; while complexes (' syllables ') are 
knowable and explicable, and you can have a true notion 
of them. So when a man gets hold of the true notion of 

c. something without an account, his mind does think truly 
of it, but he does not know it ; for if one cannot give and 
receive an account of a thing, one has no knowledge of that 
thing. But when he has also got hold of an account, all 
this becomes possible to him and he is fully equipped with 

Does that version represent the dream as you heard it, 
or not ? 
Theaet. Perfectly. 

The theory here put forward was certainly never held by Plato 
himself. On the other hand, it is obviously a philosophic theory, 

1 aroixcia meant letters of the alphabet, or the ' rudiments ' of a subject. 
This is said to be its first occurrence as applied to the elements of physical 
things. Presently avAAd/fai (syllables) is used for the complex things composed 
of elements. 

1 Buttmann's conjecture to 'to' for rovro (here and at 205c) may be sup- 
ported by Soph. 239A. See note there (p. 207). 


THEAETETUS 201c-202c 

which would not occur to common sense. It must belong to some 
contemporary of Socrates or Plato, whom Plato does not choose to 
name. 1 Possibly, Socrates is represented as ' dreaming ' it because 
the theory was really advanced after his death. There seems to be 
no evidence sufficient to identify the author. 2 

The theory may be considered under the three heads : (a) Things ; 
(b) Language ; (c) Cognition. 

(a) Things. — The only things recognised are ' ourselves and every- 
thing else ', i.e. concrete individual natural objects. These are 
composed of simple unanalysable elements. There is no question 
of immaterial things, for the elements are said to be perceptible. 
This also shows that atoms are not intended. Since no examples 
are given, we cannot say whether ' elements ' means simple primary 
substances, such as gold, or simple qualities, like yellow, or even 
whether the author drew this distinction. He may have meant any 
simple constituent that we should name in enumerating all the parts 
we can perceive and distinguish in a complex thing. 

(b) Language. — The element, being simple, has a name only. We 
can refer to or indicate it by this name. But it ' has no logos '. 
This appears to cover two meanings which we should distinguish, 
(i) We cannot make any statement about the element, such as that 
it exists. If we are to speak of it alone, we must not add, or ascribe, 
to it any second ' name ' (word). The element is completely in- 
dicated by uttering the single word ' gold ' or ' yellow \ We may 
not even say ' this is yellow ', since ' this ' and ' is ' express some- 
thing different from the simple name ' yellow ', which already 
expresses all there is to be expressed and all that I perceive. Also, 
1 this ' and ' is ' do not belong exclusively to the element I now 
perceive. (2) The name of an element is indefinable, just as the 
element itself is unanalysable. The nature is simple and no 
' account ' consisting of several names (words) can be given of it. 

The definition of logos as a ' combination of names (words) ' will 
cover statements about a thing as well as the definition of a definable 
name. But probably the author was not thinking about defining 
names (which he would not rank among complex ' things ') but only 
about describing things. The simple name indicates the elementary 

1 Theaetetus (at 201c) and Socrates (202E, rov chopra) both speak of the 
author in the singular. 

1 The case for Antisthenes was most fully stated by Gillespie (Arch. Gesch. 
Philos. xxvi, 479 ff. ; xxvii, 17 ft.). See also Ross, Metaph. of Aristotle i, 346. 
A. Levi (Revue Hist. Philos. 1930, pp. 16 ff.), among others, has disputed this 
attribution. Prof. G. C. Field has given a judicious account of Antisthenes 
in Plato and His Contemporaries (1930), 160 ff. I can see little resemblance 
between the doctrine and the atomism attributed to Ecphantus, who if 
•suggested by Burnet and Prof. Taylor. 



part ; the full description or ' account ' of a complex thing consists 
of as many names as there are elements. All statements about 
the thing he would regard as giving it names, each of which should 
belong to one of its parts. In the Sophist (p. 253) we shall meet 
again with this view of what was later called ' predication \ The 
effect is that the distinction between the definition and other state- 
ments about the thing is not drawn ; and this appears to be the 
case in our passage. 

(c) Cognition. — The theory distinguishes between perception 
(aladrjaig), a true notion (d?ir]dr]<; dotja), and knowledge (ijaarrj^rj). 

Of the element we have only a simple direct perception, not 
' knowledge \ Of the complex thing we have at first a true notion 
{aXrjBrjq do^a) without a logos. Logos, as the later argument shows, 
means enumerating by name the simple components of the complex. 
When I have done this, I have ' given an account ' of the complex 
thing and am now said to ' know 'it. I have expressed what the 
thing is by giving a list of all its simple parts. But it is hard to 
be sure what is meant by the ' true dotja ' which I have before I 
enumerate the parts. Presumably it means a complex unanalysed 
presentation of the whole object. In defence of the translation 
' true notion ' it may be remarked that Plato uses the phrase ' get 
hold of the true dd£a of a thing without a logos \ x ' Notion ' or ' im- 
pression ' seems to be meant. It may be conjectured that such a 
notion would be expressed by a definable name, such as ' man ', 
or (to use Socrates' later illustration, 207A) ' wagon \ Possibly doija 
includes the judgment ' That is a man '. This judgment may be 
true (perhaps, must be true) ; but I shall not have knowledge till I 
have enumerated all the parts of the object, which is the same thing 
as defining the name. 

The theory mentions only true notions, not false ones. It is not 
unlikely that the author held that every notion is true. If the 
notion is composed of simple perceptions, each of which is an 
impression directly given by some simple property of the thing, 
and if there can be no error in the perceptions, there can be none 
in the complex notion. The theory may hold that there must 
be just that thing I perceive or have a notion of ; otherwise I 
should be perceiving something else or nothing at all. It is quite 
possible that the author of the theory agreed (as Antisthenes 
did) with those who denied the possibility of false beliefs and 

1 202B : orav avev Xoyov r-qv aX-qdrj oo£av rivos ti? AajSfl. We have already 
noted (p. 119) Plato's use of So£a£civ with an accusative for 'thinking of a thing \ 
Again exa>v oo£av nepl aov (209A, 1) and ok eSo£a£ov (209B, 2) are used inter- 
changeably for ' having a notion of you '. 


THEAETETUS 202c-206c 

2020206c. The Theory criticised for making Elements unknowable 
For the understanding of the following argument, it is essential 
to grasp that the theory is materialistic, in the sense that the only 
1 things ' it recognises as the objects of any sort of cognition are 
concrete individual things, and the perceptible parts of which such 
things are aggregates. 

Socrates first disposes of the theory on its own ground, where the 
statement that elements are unknowable proves fatal. 

202C. Socr. So this dream finds favour and you hold that a true 
notion with the addition of an account is knowledge ? 
Theaet. Precisely. 

d. Socr. Can it be, Theaetetus, that, all in a moment, we have 
found out to-day what so many wise men have grown old in 
seeking and have not found ? 

Theaet. I, at any rate, am satisfied with our present state- 
ment, Socrates. 

Socr. Yes, the statement just in itself may well be satisfac- 
tory ; for how can there ever be knowledge without an 
account and right belief ? 1 But there is one point in the 
theory as stated that does not find favour with me. 
Theaet. What is that ? 
Socr. What might be considered its most ingenious 

e. feature : it says that the elements are unknowable, but 
whatever is complex (' syllables ') can be known. 
Theaet. Is not that right ? 

Socr. We must find out. We hold as a sort of hostage for 
the theory the illustration in terms of which it was stated. 
Theaet. Namely ? 

Socr. Letters — the elements of writing — and syllables. 
That and nothing else was the prototype the author of this 
theory had in mind, don't you think ? 
Theaet. Yes, it was. 
203. Socr. Let us take up that illustration, then, and put it to 
the question, or rather put the question to ourselves : did 
we learn our letters on that principle or not ? 2 To begin 
with : is it true that an account can be given of syllables, 
but not of letters ? 
Theaet. It may be so. 

1 This may mean that the formula ' true belief with an account ' is a satis- 
factory description at least of some knowledge, provided that the right 
meaning be given to logos, not any of the meanings discussed in the following 

1 Socrates goes back to this question at 206A. 



203. Socr. I agree, decidedly. Suppose you are asked about 
the first syllable of ' Socrates ' : ' Explain, Theaetetus ; what 
is SO ? ' How will you answer ? 
Theaet. S and O. 

Socr. And you have there an account of the syllable ? 
Theaet. Yes. 
b. Socr. Go on, then ; give me a similar account of S. 
Theaet. But how can one state the elements of an 
element ? The fact is, of course, Socrates, that S is one of 
the consonants, nothing but a noise, like a hissing of the 
tongue ; while B not only has no articulate sound but is 
not even a noise, and the same is true of most of the letters. 
So they may well be said to be inexplicable, when the clearest 
of them, the seven vowels themselves, have only a sound, 
and no sort of account can be given of them. 1 
Socr. So far, then, we have reached a right conclusion 
about knowledge. 
Theaet. Apparently. 

The ' right conclusion ' is that, if logos means an account or 
explanation consisting in the enumeration of the components of a 
complex thing, we must finally reach simple parts which cannot be 
so ' explained '. (So in mathematics the ultimate terms used in 
definitions must be indefinable.) But if such analysis is to yield 
knowledge, these ultimate components must be knowable. The 
weak point of the theory is that it says they are unknowable, and 
can only be perceived. So the process of acquiring knowledge 
will be a process of analysing a complex which is not yet known 
into components which cannot be known. 

The argument exposing this weakness is in the form of a dilemma. 
A syllable (complex) must be either (1) the mere aggregate of the 
letters, or (2) a single entity which comes into being when the 
letters are combined and vanishes when they are separated. 
Socrates easily disposes of the first alternative. 

203c. Socr. But now, have we been right in declaring that the 
letter cannot be known, though the syllable can ? 
Theaet. That seems all right. 

Socr. Take the syllable then : do we mean by that both 
the two letters or (if there are more than two) all the letters ? 

1 At Philebus i8b we find the same classification : (1) vowels (favyevra) , 
(2) consonants {a<j>a>va, with out articulate sound), (3) mutes {a<j>0oyya, which are 
not even noises). 


THEAETETUS 202c-206c 

203c. Or do we mean a single entity that comes into existence 
from the moment when they are put together ? 
Theaet. I should say we mean all the letters. 
Socr. Then take the case of the two letters S and O. 
The two together are the first syllable of my name. Anyone 
who knows that syllable knows both the letters, doesn't he ? 

d. Theaet. Naturally. 

Socr. So he knows the S and the O. 

Theaet. Yes. 

Socr. But has he, then, no knowledge of each letter, so that 

he knows both without knowing either ? 

Theaet. That is a monstrous absurdity, Socrates. 

Socr. And yet, if it is necessary to know each of two things 

before one can know both, he simply must know the letters 

first, if he is ever to know the syllable ; and so our fine theory 

will vanish and leave us in the lurch. 

e. Theaet. With a startling suddenness. 

Socr. Yes, because we are not keeping a good watch upon 

This argument is not verbal, but quite fair. If the syllable is 
exactly the same thing as its two letters, then to know the syllable 
is to know the letters. It may be added that the theory distin- 
guished knowledge from perception, and evidently regarded know- 
ledge as superior. Since the syllable is nothing more than the 
aggregate of the two letters, of each of which I have a perception, 
1 the addition of the account ' which was to yield knowledge can in 
fact only lead to two perceptions, side by side, of two unknowable 

(2) The second alternative — that the syllable is something other 
than the aggregate of the letters — requires some more subtle distinc- 

203E. Socr. {continues). Perhaps we ought to have assumed that 
the syllable was not the letters but a single entity that arises 
out of them with a unitary character of its own and different 
from the letters. 

Theaet. By all means. Indeed, it may well be so rather 
than the other way. 

Socr. Let us consider that. We ought not to abandon 
an imposing theory in this poor-spirited manner. 
Theaet. Certainly not. 

204. Socr. Suppose, then, it is as we say now : the syllable 
arises as a single entity from any set of letters which can 



204. be combined * ; and that holds of every complex, not only 
in the case of letters. 
Theaet. By all means. 
Socr. In that case, it must have no parts. 
Theaet. Why ? 

Socr. Because, if a thing has parts, the whole thing must 
be the same as all the parts. 

The term ' whole ' is here limited to mean a thing composed of 
parts into which it can be divided up, in such a way that the parts 
so arrived at account for the whole thing. Thus the sum of money 
called a shilling can be divided into twelve pence which completely 
represent its value. Nothing evaporates in the process of division. 
So the whole here is said to be exactly equivalent to ' all the parts '. 
Accordingly, if the syllable or complex is something over and above 
the letters, the letters will not be parts of that something (and it 
can have no other parts) ; so it will not be the ' whole \ From this 
statement we might pass straight to the conclusion (205c) : Since 
a syllable is a unitary thing, having no parts into which it can be 
analysed, it is simple, inexplicable, and unknowable for the same 
reason as the letter. This is the conclusion which completes the 
dilemma. It is fatal to the theory, if we keep to the theory's own 
assumptions. But here Socrates turns aside to meet the objection 
that a whole consisting of parts may not be simply the ' sum ' of 
those parts (to nav) or ' all the parts ' (rd ndvra), but a single entity 
arising out of them and distinct from them. It is true that even 
a jigsaw puzzle, when completed, has a unity as forming a picture, 
which disappears when the parts are separated. But Socrates is 
justified in arguing that that resulting entity is not properly 
described as ' the whole \ » It is an additional element which super- 
venes on the putting together of the parts which make the whole. 
He urges that the whole cannot be distinguished from the ' sum ', 
which itself cannot be distinguished from ' all the parts \ 

204A. Socr. {continues). Or do you say that a whole likewise 2 
is a single entity that arises out of the parts and is different 
from the aggregate of the parts ? 
Theaet. Yes, I do. 

Socr. Then do you regard the sum (to nav) as the same 
thing as the whole, or are they different ? 

1 avvapfiorrovTcov is not ' harmonious '. It means that only some letters 
will ' fit together ' to form a syllable : one of them must always be a vowel 
(Soph. 253A). Other combinations of letters, e.g. two or three consonants 
without a vowel, are impossible. 

2 ' likewise ' (koL), i.e. as well as the syllable, of which this has been said. 


THEAETETUS 202c-206c 

204B. Theaet. I am not at all clear ; but you tell me to answer 
boldly, so I will take the risk of saying they are different. 
Socr. Your boldness, Theaetetus, is right ; whether your 
answer is so, we shall have to consider. 
Theaet. Yes, certainly. 

Socr. Well, then, the whole will be different from the sum, 
according to our present view. 
Theaet. Yes. 

Socr. Well but now, is there any difference between the 
sum and all the things it includes ? For instance, when we 
say, ' one, two, three, four, five, six ', or ' twice three ' or 

c. ' three times two ' or ' four and two ' or ' three and two and 
one ', are we in all these cases expressing the same thing 
or different things ? 

Theaet. The same. 

Socr. Just six, and nothing else ? 

Theaet. Yes. 

Socr. In fact, in each form of expression we have expressed 

all the six. 1 

Theaet. Yes. 

Socr. But when we express them all, is there no sum 2 that 

we express ? 

Theaet. There must be. 

Socr. And is that sum anything else than ' six ' ? 

Theaet. No. 

d. Socr. Then, at any rate in the case of things that consist 
of a number, the words ' sum ' and ' all the things ' denote 
the same thing. 

Theaet. So it seems. 

Socr. Let us put our argument, then, in this way. The 
number of (square feet in) an acre, and the acre are the same 
thing, aren't they ? 
Theaet. Yes. 

Socr. And so too with the number of (feet in) a mile ? 
Theaet. Yes. 

Socr. And again with the number of (soldiers in) an army 
and the army, and so on, in all cases. The total number is 
the same as the total thing in each case. 
Theaet. Yes. 
E. Socr. But the number of (units in) any collection of things 
cannot be anything but parts of that collection ? 

1 Reading -navra t<x c| with BT. 

2 The word ' sum ' (nav) here is necessary to the argument. The manu- 
scripts have -ndXiv. 



204E. Theaet. No. 

Socr. Now, anything that has parts consists of parts. 

Theaet. Evidently. 

Socr. But all the parts, we have agreed, are the same as 

the sum, if the total number is to be the same as the total 


Theaet. Yes. 

Socr. The whole, then, does not consist of parts ; for if it 

were all the parts it would be a sum. 

Theaet. Apparently not. 

Socr. But can a part be a part of anything but its whole ? 

Theaet. Yes ; of the sum. 
205. Socr. You make a gallant fight of it, Theaetetus. But 

does not ' the sum ' mean precisely something from which 

nothing is missing ? 

Theaet. Necessarily. 

Socr. And is not a whole exactly the same thing — that 

from which nothing whatever is missing ? Whereas, when 

something is removed, the thing becomes neither a whole 

nor a sum : it changes at the same moment from being both 

to being neither. 

Theaet. I think now that there is no difference between 

a sum and a whole. 

Plato is not denying that there are wholes which contain an 
additional element that arises when the parts are put together and 
disappears when they are separated. He was aware of this, 1 but 
his point is that such an additional element is not what we mean 
by ' the whole \ It may also be remarked that he is arguing 
within the limits of the theory he is criticising. That theory holds 
that the only things we can perceive or know or talk about are 
concrete individual things in nature, complex or simple, and that 
a complex thing is no more than an aggregate of simple things or 
elements, which can be enumerated in the only account we can 
give of it. When the enumeration is complete we know all that 
we can know about the thing. So the whole is nothing but the 
sum of its parts. A man is, for this theory, a trunk and a head 
and limbs. There is no substance or essence ' Man ', over and above 
the separable ' material ' parts, such as Plato and Aristotle would 
recognise and make the subject of a definition (logos) by genus and 
specific difference. 

Having ruled out the suggestion that ' the whole ' can be a single 
entity distinct from all the parts, Socrates can now return to the 

1 Cf. Aristotle's discussion, inspired by the Theaetetus, at Metaph. z, 17. 


THEAETETUS 202c-206c 

argument interrupted at 204A, namely the second alternative : 
that the syllable or complex is a unity over and above its letters or 
elements. He can now reaffirm the statement there made, that if 
the syllable is such a unity, it is not a whole and can have no parts. 

205A. Socr. Well, we were saying — were we not ? — that when 
a thing has parts, the whole or sum will be the same thing 
as all the parts ? 
Theaet. Certainly. 
Socr. To go back, then, to the point I was trying to make 

b. just now ; if the syllable is not the same thing as the letters, 
does it not follow that it cannot have the letters as parts 
of itself ; otherwise, being the same thing as the letters, it 
would be neither more nor less knowable than they are ? 
Theaet. Yes. 

Socr. And it was to avoid that consequence that we sup- 
posed the syllable to be different from the letters. 
Theaet. Yes. 

Socr. Well, if the letters are not parts of the syllable, 
can you name any things, other than its letters, that are 
parts of a syllable ? 

Theaet. Certainly not, Socrates. If I admitted that it 
had any parts, it would surely be absurd to set aside the 
letters and look for parts of any other kind. 

c. Socr. Then, on the present showing, a syllable will be a 
thing that is absolutely one and cannot be divided into parts 
of any sort ? * 

Theaet. Apparently. 

Socr. Do you remember then, my dear Theaetetus, our 
accepting a short while ago a statement that we thought 
satisfactory : that no account could be given of the primary 
things of which other things are composed, because each of 
them, taken just by itself, was incomposite ; and that it 
was not correct to attribute even ' existence ' to it, or to 
call it ' this ', on the ground that these words expressed 
different things that were extraneous to it ; and this was 
the ground for making the primary thing inexplicable 
and unknowable ? 
Theaet. I remember. 

d. Socr. Then is not exactly this, and nothing else, the 
ground of its being simple in nature and indivisible into 
parts ? I can see no other. 

1 Travrairaai, put first for emphasis, should be construed with /w'a ns iSe'a 



205D. Theaet. Evidently there is no other. 

Socr. Then has not the syllable now turned out to be a 
thing of the same sort, if it has no parts and is a unitary 
thing ? 

Theaet. Certainly. 

Socr. To conclude, then : if, on the one hand, the syllable 
is the same thing as a number of letters and is a whole with 
the letters as its parts, then the letters must be neither more 
nor less knowable and explicable than syllables, since we 
made out that all the parts are the same thing as the whole. 
e. Theaet. True. 

Socr. But if, on the other hand, the syllable is a unity 
without parts, syllable and letter likewise are equally 
incapable of explanation and unknowable. The same 
reason will make them so. 
Theaet. I see no way out of that. 
Socr. If so, we must not accept this statement : that 
the syllable can be known and explained, the letter cannot. 
Theaet. No, not if we hold by our argument. 

Putting aside the illustration from letters, it has now been 
established that knowledge cannot be gained, as the theory holds, 
by analysing a concrete thing, presented in a complex notion, 
into its simple parts, each presented in a simple perception which 
is not knowledge. 

It is finally pointed out that the illustration itself tells 
against the theory. Our knowledge of letters must actually be 
clearer than our knowledge of syllables, whereas the theory 
evidently regards our perception of elements as inferior to the 
knowledge we are alleged to gain by giving an account of the 

206. Socr: And again, would not your own experience in 
learning your letters rather incline you to accept the opposite 
view ? 

Theaet. What view do you mean ? 
Socr. This : that all the time you were learning you 
were doing nothing else but trying to distinguish by sight 
or hearing each letter by itself, so as not to be con- 
fused by any arrangement of them in spoken or written 

Theaet. That is quite true. 

Socr. And in the music school the height of accomplish- 
b. ment lay precisely in being able to follow each several 



2o6b. note and tell which string it belonged to ; and notes, as 
everyone would agree, are the elements of music. 1 
Theaet. Precisely. 

Socr. Then, if we are to argue from our own experience 
of elements and complexes to other cases, we shall conclude 
that elements in general yield knowledge that is much 
clearer than knowledge of the complex and more effective 
for a complete grasp of anything we seek to know. If 
anyone tells us that the complex is by its nature knowable, 
while the element is unknowable, we shall suppose that, 
whether he intends it or not, he is playing with us. 
Theaet. Certainly. 

206C-E. Three possible meanings of ' account \ (i) Expression of 
thought in speech {irrelevant) 

The refutation of the theory ' dreamt ' by Socrates is now com- 
plete. It turns upon the allegation that the simple and unanalys- 
able is unknowable. But Theaet etus' suggestion that knowledge 
is true judgment or belief combined with an account or explanation 
may have other meanings not involving this fatal flaw. Socrates 
accordingly turns to consider these possible meanings. The 
discussion still proceeds, however, on certain assumptions of the 
refuted theory, namely that the only things to be known are con- 
crete individual things, and that knowledge accordingly must 
consist in giving some account of such things. This limitation is 
in accordance with the scope of the whole dialogue, which asks 
whether knowledge can be extracted from the world of concrete 
natural things, yielding perceptions and complex notions, without 
invoking other factors. The three meanings of logos now considered 
are determined by these assumptions, which exclude Plato's own 
view, that the objects of which knowledge must give an account 
are not concrete individuals but objects of thought, and that the 
simpler terms in which the account must be stated are not material 
parts but higher concepts. 

206c. Socr. Indeed we might, I think, find other arguments to 
prove that point. But we must not allow them to distract 
our attention from the question before us, namely, what 
can really be meant by saying that an account added to 
true belief yields knowledge in its most perfect form. 

1 The appeal to music and (earlier) to numbers and measures lends no sup- 
port to Campbell's suggestion that the theory is due to ' some Pythagorean ' 
(p. xxxix). These examples are brought forward, not by the author of the 
theory, but by Socrates in refuting it. 



206c. Theaet. Yes, we must see what that means. 

Socr. Well then, what is this term ' account ' intended 
to convey to us ? I think it must mean one of three things. 
Theaet. What are they ? 

d. Socr. The first will be giving overt expression to one's 
thought by means of vocal sound with names and verbs, 
casting an image of one's notion on the stream that 
flows through the lips, like a reflection in a mirror or in 
water. Do you agree that expression of that sort is an 
' account ' ? 

Theaet. I do. We certainly call that expressing our- 
selves in speech (Xeyetv). 

Socr. On the other hand, that is a thing that anyone 
can do more or less readily. If a man is not born deaf 
or dumb, he can signify what he thinks on any subject. 
So in this sense anyone whatever who has a correct 

e. notion evidently will have it ' with an account ', and 
there will be no place left anywhere for a correct notion 
apart from knowledge. 

Theaet. True. 

Logos here does not mean a ' verbal definition ' such as a dictionary 
gives, but simply ' statement ', ' speech ' — the utterance of the 
notion or judgment in our minds. This common meaning of the 
word is mentioned only for the sake of clearness. It is obviously 
not what Theaetetus intended. 

206E-208B. (2) Enumeration of elementary parts. This will not 
convert a true notion into knowledge 

The second meaning is the enumeration of elementary parts. 
This is now considered on its own merits, apart from the further 
feature which proved fatal to the earlier theory, namely, the 
doctrine that an element must be unknowable. 

206E. Socr. Then we must not be too ready to charge the 
author of the definition of knowledge now before us x with 
talking nonsense. Perhaps that is not what he meant. 
He may have meant : being able to reply to the question, 

207. what any given thing is, by enumerating its elements. 
Theaet. For example, Socrates ? 

Socr. For example, Hesiod says about a wagon, ' In a 
wagon are a hundred pieces of wood.' I could not name 

1 The author of the definition originally quoted by Theaetetus (20 id), who 
is now regarded as not responsible for the doctrine, in the theory ' dreamt ' 
by Socrates, that elements are unknowable. 


THEAETETUS 206e-208b 

207. them all ; no more, I imagine, could you. If we were 
asked what a wagon is, we should be content if we could 
mention wheels, axle, body, rails, yoke. 
Theaet. Certainly. 

Socr. But I dare say he would think us just as ridiculous 
as if we replied to the question about your own name by 

b. telling the syllables. We might think and express our- 
selves correctly, but we should be absurd if we fancied 
ourselves to be grammarians and able to give such an 
account of the name Theaetetus as a grammarian would 
offer. He would say it is impossible to give a scientific 
account of anything, short of adding to your true notion 
a complete catalogue of the elements, as, I think, was 
said earlier. 

Theaet. Yes, it was. 

Socr. In the same way, he would say, we may have a 
correct notion of the wagon, but the man who can give a 
complete statement of its nature by going through those 

c. hundred parts has thereby added an account to his correct 
notion and, in place of mere belief, has arrived at a technical 
knowledge of the wagon's nature, by going through all 
the elements in the whole. 

Theaet. Don't you approve, Socrates ? 
Socr. Tell me if you approve, my friend, and whether you 
accept the view that the complete enumeration of elements 
is an account of any given thing, whereas description in 
terms of syllables or of any larger unit still leaves it un- 

d. accounted for. Then we can look into the matter further. 
Theaet. Well, I do accept that. 

Socr. Do you think, then, that anyone has knowledge 

of whatever it may be, when he thinks that one and the 

same thing is a part sometimes of one thing, sometimes 

of a different thing ; or again when he believes now one 

and now another thing to be part of one and the same thing ? 

Theaet. Certainly not. 

Socr. Have you forgotten, then, that when you first began 

learning to read and write, that was what you and your 

schoolfellows did ? 

Theaet. Do you mean, when we thought that now one 

e. letter and now another was part of the same syllable, and 
when we put the same letter sometimes into the proper 
syllable, sometimes into another ? 

Socr. That is what I mean. 

Theaet. Then I have certainly not forgotten ; and I do 



207E. not think that one has reached knowledge so long as one 

is in that condition. 

Socr. Well then, if at that stage you are writing ' Theae- 

tetus ' and you think you ought to write T and H and E 

and do so, and again when you are trying to write ' Theo- 
208. dorus ', you think you ought to write T and E and do so, 

can we say that you know the first syllable of your two 

names ? 

Theaet. No ; we have just agreed that one has not 

knowledge so long as one is in that condition. 

Socr. And there is no reason why a person should not 

be in the same condition with respect to the second, third, 

and fourth syllables as well ? 

Theaet. None whatever. 

Socr. Can we, then, say that whenever in writing ' Theae- 

tetus ' he puts down all the letters in order, then he is in 

possession of the complete catalogue of elements together 

with correct belief ? 

Theaet. Obviously. 
b. Socr. Being still, as we agree, without knowledge, though 

his beliefs are correct ? 

Theaet. Yes. 

Socr. Although he possesses the ' account ' in addition 

to right belief. For when he wrote he was in possession 

of the catalogue of the elements, which we agreed was the 

1 account \ 

Theaet. True. 

Socr. So, my friend, there is such a thing as right belief 

together with an account, which is not yet entitled to be 

called knowledge. 

Theaet. I am afraid so. 

Socr. Then, apparently, our idea that we had found the 

perfectly true definition of knowledge was no better than 

a golden dream. 

Socrates has now disposed of the theory that the addition of a 
complete enumeration of elements to a correct, but previously 
unanalysed, notion of a complex thing will convert true belief 
into knowledge. Even if we reject the doctrine that the element 
is unknowable, and suppose it to be at least as knowable as the 
complex, still the complete enumeration may fail to give us any- 
thing better than true belief. The analysis, though it be carried 
as far as possible, will not yield knowledge of any different kind 
from the true notion we started with, or the correct beliefs about 


THEAETETUS 208b-210b 

the parts of a wagon which stopped short at five parts instead 
of all the hundred. So the schoolboy may have a correct belief 
about every letter in the name ' Theaetetus ' and write it correctly, 
without having that assured knowledge which would save him from 
writing it incorrectly on another occasion. 

If we go behind the illustration and beyond the limits of the 
theory that is being criticised, we see further into Plato's mind. 
In the Meno the slave who is ignorant of geometry is led through 
a problem till he reaches the correct solution. But Socrates points 
out that he still has only true belief, not knowledge, because he 
does not understand the proof or see how the conclusion neces- 
sarily follows from the premisses. Even if he were taken back 
through the earlier propositions, axioms, and definitions to the 
primitive indefinables, he might still possess no more than an 
exhaustive catalogue of true beliefs leading to the solution. He 
will not know even this much of geometry until he has grasped the 
necessary connexion which will make all these beliefs abiding and 
unshakable. All this, however, lies outside the presuppositions 
of the theory under examination, which contemplates only the 
analysis of a concrete thing into elementary parts. 

208B-210B. (3) The statement of a distinguishing mark. This will 
not convert a true notion into knowledge 

Socrates now suggests a third possible meaning of logos — ' being 
able to state some mark by which the thing in question differs 
from everything else \ Will this addition convert true belief into 
knowledge ? Logos will now mean the ' account ' of a thing given 
by a description which serves to distinguish the thing we wish to 
indicate from all other things. 

208B. Socr. [continues). Or shall we not condemn the theory 
c. yet ? Perhaps the meaning to be given to ' account ' is 
not this, but the remaining one of the three, one of which 
we said must be intended by anyone who defines knowledge 
as correct belief together with an account. 
Theaet. A good reminder ; there is still one meaning 
left. The first was what might be called the image of 
thought in spoken sound ; and the one we have just dis- 
cussed was going all through the elements to arrive at the 
whole. What is the third ? 

Socr. The meaning most people would give : being able 
to name some mark by which the thing one is asked about 
differs from everything else. 

Theaet. Could you give me an example of such an account 
of a thing ? 



2o8d. Socr. Take the sun as an example. I dare say you will 
be satisfied with the account of it as the brightest of the 
heavenly bodies that go round the earth. 
Theaet. Certainly. 

Socr. Let me explain the point of this example. It is 
to illustrate what we were just saying : that if you get 
hold of the difference distinguishing any given thing from 
all others, then, so some people say, you will have an 
' account ' of it ; whereas, so long as you fix upon some- 
thing common to other things, your account will embrace 
all the things that share it. 
e. Theaet. I understand. I agree that what you describe 
may fairly be called an ' account \ 

Socr. And if, besides a right notion about a thing, what- 
ever it may be, you also grasp its difference from all other 
things, you will have arrived at knowledge of what, till 
then, you had only a notion of. 
Theaet. We do say that, certainly. 
Socr. Really, Theaetetus, now I come to look at this 
statement at close quarters, it is like a scene-painting : 
I cannot make it out at all, though, so long as I kept at 
a distance, there seemed to be some sense in it. 
Theaet. What do you mean ? Why so ? 

209. Socr. I will explain, if I can. Suppose I have a correct 
notion about you ; if I add to that the account of you, 
then, we are to understand, I know you. Otherwise I have 
only a notion. 
Theaet. Yes. 

Socr. And ' account ' means putting your differentness 1 
into words. 
Theaet. Yes. 

Socr. So, at the time when I had only a notion, my 
mind did not grasp any of the points in which you differ 
from others ? 

Theaet. Apparently not. 

Socr. Then I must have had before my mind one of those 
common things which belong to another person as much 
as to you. 
b. Theaet. That follows. 

Socr. But look here ! If that was so, how could I possibly 

1 Plato seems deliberately to avoid the term 8ia<f>opd here and henceforward 
(though it occurred at 208D), perhaps because of its technical use for the 
differentia of a species, which is irrelevant to this context. AiafopoTrjs is a 
Platonic word which occurs again at Rep. 587E. 


THEAETETUS 208b-210b 

209B. be having a notion of you rather than of anyone else ? 
Suppose I was thinking : Theaetetus is one who is a man 
and has a nose and eyes and a mouth and so forth, enumer- 
ating every part of the body. Will thinking in that way 
result in my thinking of Theaetetus rather than of Theo- 
doras or, as they say, of the man in the street ? 
Theaet. How should it ? 
Socr. Well, now suppose I think not merely of a man 

c. with a nose and eyes, but of one with a snub nose and 
prominent eyes, once more shall I be having a notion of 
you any more than of myself or anyone else of that descrip- 
tion ? 

Theaet. No. 

Socr. In fact, there will be no notion of Theaetetus in 
my mind, I suppose, until this particular snubness has 
stamped and registered within me a record distinct from 
all the other cases of snubness that I have seen ; and so 
with every other part of you. Then, if I meet you to- 
morrow, that trait will revive my memory and give me 
a correct notion about you. 
Theaet. Quite true. 

d. Socr. If that is so, the correct notion of anything must 
itself include the differentness of that thing. 

Theaet. Evidently. 

Socr. Then what meaning is left for getting hold of an 
' account ' in addition to the correct notion ? If, on the 
one hand, it means adding the notion of how a thing differs 
from other things, such an injunction is simply absurd. 
Theaet. How so ? 

Socr. When we have a correct notion of the way in which 
certain things differ from other things, it tells us to add a 
correct notion of the way in which they differ from other 
E. things. On this showing, the most vicious of circles would 
be nothing to this injunction. It might better deserve to 
be called the sort of direction a blind man might give : to 
tell us to get hold of something we already have, in order 
to get to know something we are already thinking of, sug- 
gests a state of the most absolute darkness. 

Theaet. Whereas, if ? The supposition you made 

just now implied that you would state some alternative ; 
what was it ? * 

1 Reading tl be ye — rt vvvb-q o»? eptov <^ert^> v-nedov ; The objection to reading 
(with Burnet and others) cine brj rt vvvb-q cjs epwv envdov is that Socrates' last 
question (to ovv rrpoaXa^elv . . . cftj ; 209D, 4) did not suggest that he had 



209E. Socr. If the direction to add an ■ account ' means that 
we are to get to know the differentness, as opposed to 
merely having a notion of it, this most admirable of all 
definitions of knowledge will be a pretty business ; because 

210. ' getting to know ' means acquiring knowledge, doesn't it ? 
Theaet. Yes. 

Socr. So, apparently, to the question, What is knowledge ? 
our definition will reply : ' Correct belief together with 
knowledge of a differentness ' ; for, according to it, ' adding 
an account ' will come to that. 
Theaet. So it seems. 

Socr. Yes ; and when we are inquiring after the nature 
of knowledge, nothing could be sillier than to say that it 
is correct belief together with a knowledge of differentness 
or of anything whatever. 

So, Theaetetus, neither perception, nor true belief, nor 
b. the addition of an ' account ' to true belief can be knowledge. 
Theaet. Apparently not. 

Some critics have imagined that the above argument is con- 
cerned with the definition of species by genus and specific differ- 
ence, and even that Plato is here criticising himself. But it is 
clearly presumed throughout that the object to be defined and 
known is a concrete individual thing — ' ourselves and other things ', 
Hesiod's wagon, a person (Theaetetus), the sun. The ' different- 
ness ' is a perceptible individual peculiarity, such as ' this par- 
ticular snubness which I have seen ', distinguishing this individual 
person from other individuals, not a specific difference distinguish- 
ing a species from other species and common to all individuals of 
the species. 

something more to say. What did suggest this was the el fiev (209D, 5), 
implying that an alternative supposition (ct 8e) was to follow — the supposition 
stated in Socrates' next speech (et to Xoyov . . . 209E, 6). Badham saw 
this and tried to restore the necessary sense to Theaetetus' inquiry by reading 
ci 84 ye — ti vi>v8^ a*? crcpov vnedov ; ' Whereas if — what was it you suggested 
just now as the alternative ? ' The sense is better, if it could be got out of 
the words. But (as Campbell noted) vnorideadat., though it can mean to 
put an explicit suggestion to a person, cannot mean to imply something not 
stated at all ; and the imperfect would be required. 

The reading I propose (Class. Rev. xliv (1930), 114) means: ' Whereas 
if — what was it (the " whereas if ") that your supposition just now (" if on 
the one hand ") implied (a>?) that you were going on to state ? ' For elnctp 
€ti, ci. Soph., O.T. 748, Sei^et? 8e [x&XXov, rjv €v €^Ltttjs !ti. 

The rather obscure form of the question is (like the rest of these concluding 
pages) in the manner of the Sophist ; e.g. 217A, ri 8c ^dXtara kox to ttoZov ti -nepl 
avru>v SiaTTop-qdels ipeaOai hievorjOrjS ; 226c, to ttoiov avrcov iripi (SovXtjOcIs SrjXojaat 
Trapahciyp-ara npodels ravra Kara -navraiv rjpov ; 


THEAETETUS 208b-210b 

Socrates argues : Suppose I have a correct notion of Theaetetus. 
If my notion contains only traits he shares with all or some other 
men, then it is not a notion of him any more than of them. It 
must include his individual and peculiar characteristics. Thus 
my notion of his individual ' differentness ' is already included in 
my notion of just that person, and I am acquainted with that 
differentness in just the same way as I am with his common char- 
acteristics. It is absurd to tell me to add it to my notion of the 
person as a whole or to suppose that such an addition could con- 
vert a correct notion into some higher kind of cognition called 
' knowledge \ 

The instance of the sun recalls Aristotle's argument that it is 
impossible to define an individual sensible substance. 1 A definition 
must consist of words whose established meanings can all apply 
to other actual or possible individuals. Even if you take an 
eternal substance which is in fact unique, such as the sun or moon, 
it is still impossible to define it. Some attributes of the sun (going 
round the earth, invisible at night) might be removed, and yet 
the sun would still be the sun. Any description such as ' the 
brightest of the heavenly bodies ' must consist of attributes that 
might belong to another subject. There can, at any time, be only 
one body which is ' the brightest ', but if a brighter body should 
appear in the heavens, the description would transfer itself to that. 

There is no question here of the definition of species, which 
are definable precisely because no two species are conceptually 
identical, as any number of individuals may be. The whole dis- 
cussion is confined to the level of the theory ' dreamt ' by Socrates, 
which contemplates only our acquaintance with individual sensible 
things. The point is that we cannot get ' knowledge ', supposed 
to be somehow superior to mere beliefs or notions, by adding a 
logos in any of the senses considered. These senses appear to 
exhaust the possible ways in which an ' account ' can be given 
of an individual thing, (i) We may name it (express our notion 
of it in speech) ; (2) we may enumerate the material parts of 
which it is composed ; or (3) we may point it out by a description 
which will serve to distinguish the thing we indicate from other 
things. But none of these ' accounts ' will yield any ' clearer ' 
or more certain kind of cognition than we started with. 

The Platonist will draw the necessary inference. True know- 
ledge has for its object things of a different order — not sensible 
things, but intelligible Forms and truths about them. Such objects 
are necessarily unique ; they do not become and perish or change 

1 Metaph. z, 15. Aristotle took the example of the Sun from our passage 
and evidently understood Plato's meaning correctly. 



in any respect. Hence we can know them and eternal truths 
about them. The Theaetetus leads to this old conclusion by demon- 
strating the failure of all attempts to extract knowledge from 
sensible objects. 

210B-D. Epilogue. All these attempts to define knowledge have 

It only remains to point out that all these attempts have failed 
and no others are forthcoming. 

210B. Socr. Are we in labour, then, with any further child, my 
friend, or have we brought to birth all we have to say about 
knowledge ? 

Theaet. Indeed we have ; and for my part I have already, 
thanks to you, given utterance to more than I had in me. 
Socr. All of which our midwife's skill pronounces to be 
mere wind-eggs and not worth the rearing ? 
Theaet. Undoubtedly. 
Socr. Then supposing you should ever henceforth try to 

c. conceive afresh, Theaetetus, if you succeed, your embryo 
thoughts will be the better as a consequence of to-day's 
scrutiny ; and if you remain barren, you will be gentler and 
more agreeable to your companions, having the good sense 
not to fancy you know what you do not know. For that, 
and no more, is all that my art can effect ; nor have I 
any of that knowledge possessed by all the great and admir- 
able men of our own day or of the past. But this midwife's 
art is a gift from heaven ; my mother had it for women, 

d. and I for young men of a generous spirit and for all in 
whom beauty dwells. 1 

Now I must go to the portico of the King Archon to meet 
the indictment which Meletus has drawn up against me. 
But to-morrow morning, Theodorus, let us meet here again. 

1 KaAot refers to beauty of mind, such as Theaetetus has, rather than bodily 
beauty. Cf. 185E. 



University of